Ovinomancer
No flips for you!
Okay, so, to me, the crux of a check is that the fiction changes after it's made. On a success, the fiction changes towards the goal. On a failure, the fiction changes away. The away part is the consequence for failure, and take many forms, but must always, always, always be a change in the fiction.
So, to that end, if you're having a bit of trouble coming up with whether a change should happen, get formal with your stake setting. Before you ask for a roll, clarify what you think the stated goal and approach is, say what you think a success looks like and then what a failure looks like, then set a DC. If you get to the middle and can't come up with what a failure looks like, don't ask for the roll, just award the success. If you get to the DC and think, "oh, I'll set this at 30!" Don't ask for the roll, narrate the failure. The trick is to get in the habit of thinking "what changes with this roll" every time. If you need to do this out loud and formally, go for it -- it won't make your game worse. I prefer explicit stake setting, and it helps me remember because I, too, still occasionally wrestle with old habits.
As for knowledge checks -- these are hard because they don't function like other ability checks in the 5e rules. They're kinda vague, and don't seem to require a check, and go into telling the players what their characters think. Failure cases seems to be either "you don't know" or making up wrong information. I dislike the wrong info path, as it may erode trust in the DM's provision of information, which is critical for a good game. To get around this, I tend to make knowledge style checks active. If you want info, you can have it if you're appropriately trained. "I recall what I know about demons to identify this one and determine it's weaknesses" gets a pretty direct answer if you leverage "using my long training in planes under my Arcana Masters" or "because I grew up hiding from them in the Abyss as part of my planar urchin background." Whatevs. I do not predicate my challenges on not knowing things about the foe, so it's not a big deal to provide this kind of information freely.
What I try to do with these skills, though, is provide framing that encourages active use of the skills. I may say, "this demon is one you've not discussed in classes or study before, but given the depths of the Infinite Abyss and it's chaotic nature, that's to be expected. You do recall the taxonomy of demons from the lectures, though, so you can ask one question for free from your study or it's a DC 15 INT check. Succeed and you get 3 questions. Fail and you still get the question, but you'll grant advantage to the demon as you get momentarily distracted by your study." Or, maybe it's disadvantage on a save. Depends. Either way, I make the check an active interaction with the environment and this helps ground the fictional positioning for success and failure.
This is a poor example, though, as I usually don't do trick monster encounters like this. I do build in plenty of features where use of the "knowledge" skills turns into direct activity. I try to give what are otherwise single niche challenges a broader appeal by grounding them into the fiction a bit more. Say, maybe, you'd have a trap on a chest that they party needs to get the stuff inside while there's a fight going on (otherwise, traps aren't as much fun). Instead of just a challenge for the rogue, maybe provide that the trap is based on an ancient society and is, in fact, a puzzle based on that society's legends. Then the cleric who's trained in history can jump onto solving the trap by leveraging that while the rogue gets to go fight for a bit. I try to do this often -- make situations have more than one obvious hook with one often tying into a 'knowledge' type skill so that these get screentime in ways that aren't "is it okay if my character knows something?' and more in the 'My character knows something, and will use it to do this thing." In other words, design so that you lean on the characters having knowledge and use the knowledge skills that way rather than just use them to ask questions. This both increases the depth of the fiction by becoming more rich but also lets players that take knowledge skills feel like that skill is something that says how cool their character is rather than a button to press to get information from the DM.
I do the same with Insight -- I use it as the "elicit information" skill. Persuade is for convincing others, intimidate for scaring them into compliance, and deception is for... deceiving. Insight is for find out things. If you build your social challenges such that the NPCs have handles like I suggest for the traps above, then Insight is super engaging as a skill that gives you a leg up or a different in to this NPC rather than just punching the CHA skills. Don't make "Is this NPC lying to me" a part of the challenge -- make why the NPC is lying, or what specifically they're lying about the challenge. If you do this, then Insight isn't the button mash "are they lying skill" because your'e telegraphing the deception because your challenge isn't about detecting a lie, it's what you do anyway. Then hang some stuff on the NPC -- a bond or flaw (you don't have to do all of them, one or two usually does for most NPCs and makes them richer in play) that the players can Insight to pick up on and then leverage. Use the social rules in the DMG -- they actually work pretty well and create a deeper interaction. Don't just freeplay social interactions, formalize them a bit. Get explicit with the players. I'm working with my group and fighting my bad acting habits to get social interactions further away from us acting and someone dominating the social roleplay through superior acting to making the social part of the game actually part of the game. The funny voices are still there, they're just no longer standing in for the resolution mechanics.
All in all, to steal from PbtA, be a fan of the PCs. This doesn't mean give them stuff, but instead means be interested in who they are and what they do. Give them interesting things to interact with so you can enjoy the characters. But, don't shy away from laying on the hurt, because they're even more interesting in adversity, and bringing adversity is your job as DM.
So, to that end, if you're having a bit of trouble coming up with whether a change should happen, get formal with your stake setting. Before you ask for a roll, clarify what you think the stated goal and approach is, say what you think a success looks like and then what a failure looks like, then set a DC. If you get to the middle and can't come up with what a failure looks like, don't ask for the roll, just award the success. If you get to the DC and think, "oh, I'll set this at 30!" Don't ask for the roll, narrate the failure. The trick is to get in the habit of thinking "what changes with this roll" every time. If you need to do this out loud and formally, go for it -- it won't make your game worse. I prefer explicit stake setting, and it helps me remember because I, too, still occasionally wrestle with old habits.
As for knowledge checks -- these are hard because they don't function like other ability checks in the 5e rules. They're kinda vague, and don't seem to require a check, and go into telling the players what their characters think. Failure cases seems to be either "you don't know" or making up wrong information. I dislike the wrong info path, as it may erode trust in the DM's provision of information, which is critical for a good game. To get around this, I tend to make knowledge style checks active. If you want info, you can have it if you're appropriately trained. "I recall what I know about demons to identify this one and determine it's weaknesses" gets a pretty direct answer if you leverage "using my long training in planes under my Arcana Masters" or "because I grew up hiding from them in the Abyss as part of my planar urchin background." Whatevs. I do not predicate my challenges on not knowing things about the foe, so it's not a big deal to provide this kind of information freely.
What I try to do with these skills, though, is provide framing that encourages active use of the skills. I may say, "this demon is one you've not discussed in classes or study before, but given the depths of the Infinite Abyss and it's chaotic nature, that's to be expected. You do recall the taxonomy of demons from the lectures, though, so you can ask one question for free from your study or it's a DC 15 INT check. Succeed and you get 3 questions. Fail and you still get the question, but you'll grant advantage to the demon as you get momentarily distracted by your study." Or, maybe it's disadvantage on a save. Depends. Either way, I make the check an active interaction with the environment and this helps ground the fictional positioning for success and failure.
This is a poor example, though, as I usually don't do trick monster encounters like this. I do build in plenty of features where use of the "knowledge" skills turns into direct activity. I try to give what are otherwise single niche challenges a broader appeal by grounding them into the fiction a bit more. Say, maybe, you'd have a trap on a chest that they party needs to get the stuff inside while there's a fight going on (otherwise, traps aren't as much fun). Instead of just a challenge for the rogue, maybe provide that the trap is based on an ancient society and is, in fact, a puzzle based on that society's legends. Then the cleric who's trained in history can jump onto solving the trap by leveraging that while the rogue gets to go fight for a bit. I try to do this often -- make situations have more than one obvious hook with one often tying into a 'knowledge' type skill so that these get screentime in ways that aren't "is it okay if my character knows something?' and more in the 'My character knows something, and will use it to do this thing." In other words, design so that you lean on the characters having knowledge and use the knowledge skills that way rather than just use them to ask questions. This both increases the depth of the fiction by becoming more rich but also lets players that take knowledge skills feel like that skill is something that says how cool their character is rather than a button to press to get information from the DM.
I do the same with Insight -- I use it as the "elicit information" skill. Persuade is for convincing others, intimidate for scaring them into compliance, and deception is for... deceiving. Insight is for find out things. If you build your social challenges such that the NPCs have handles like I suggest for the traps above, then Insight is super engaging as a skill that gives you a leg up or a different in to this NPC rather than just punching the CHA skills. Don't make "Is this NPC lying to me" a part of the challenge -- make why the NPC is lying, or what specifically they're lying about the challenge. If you do this, then Insight isn't the button mash "are they lying skill" because your'e telegraphing the deception because your challenge isn't about detecting a lie, it's what you do anyway. Then hang some stuff on the NPC -- a bond or flaw (you don't have to do all of them, one or two usually does for most NPCs and makes them richer in play) that the players can Insight to pick up on and then leverage. Use the social rules in the DMG -- they actually work pretty well and create a deeper interaction. Don't just freeplay social interactions, formalize them a bit. Get explicit with the players. I'm working with my group and fighting my bad acting habits to get social interactions further away from us acting and someone dominating the social roleplay through superior acting to making the social part of the game actually part of the game. The funny voices are still there, they're just no longer standing in for the resolution mechanics.
All in all, to steal from PbtA, be a fan of the PCs. This doesn't mean give them stuff, but instead means be interested in who they are and what they do. Give them interesting things to interact with so you can enjoy the characters. But, don't shy away from laying on the hurt, because they're even more interesting in adversity, and bringing adversity is your job as DM.