Actually, I wanted you to be even
more specific. Like, what is the setup? What's the NPC lying about? Why? But, ok, you made it much easier than that. (If requiring more typing, with 4 scenarios...)
Not entirely directed at you, but this has been a recurring issue.
Scenario 1:
The NPC is telling the truth. The player (for whatever reason) is suspicious.
How I'd handle it:
DM as NPC: "I don't know anything about the missing jewels"
Player: "I don't believe him. " Rolling dice "I get an 18 insight"
or: "I don't believe him, can I roll an insight?"
or: "I don't believe him, does he look like he's trying to be deceptive?"
DM: ask for an insight roll if they didn't already give one. DM rolls dice and ignores result responding "They seem to be telling the truth"
In this scenario I don't care how the player declares what they're trying to do. The intent is clear. If they roll high enough I may give them some additional info that the NPC is nervous about something but not necessarily lying. Low enough? Maybe they believe the NPC is lying. Depends on specifics.
Scenario: they're talking to the estranged sister of an NPC they're trying to find, who claims not to have seen him in years.
Clue: The DM let the heroes find a letter she wrote to him only months ago.
NPC: "No, I haven't seen him in 11 years, ever since (fill in backstory)"
Player (using clue): "And you haven't tried to contact him in all this time?"
NPC: "Oh, I've tried to contact him, all right. Just a few months ago I sent a letter to an inn I know he used to frequent, hoping it would get to him. Never heard anything back."
The fact that she didn't try to hide the existence of the letter should be a strong hint she isn't lying. And now they have another clue, by asking her which inn she sent the letter to.
If the players get stuck, a successful Insight check might prod them with a clue to the clue. "You notice she hasn't said anything about trying to find him, just that she hasn't heard from him."
EDIT: And I'll add that [MENTION=97077]iserith[/MENTION] is entirely right: the other way you could do this is to generate bonds/flaws/ideals/theoneI'mforgetting and on successful Insight checks reveal those, and should provide hints as to the situation.
Scenario 2:
The NPC is lying.
See above with appropriate results. I never say "they're lying" it would be "they don't seem to be telling the truth" or "they seem to be hiding something. Failed insight? "They seem to be telling the truth".
"Why would I try to contact that ungrateful wretch?"
(Again, Insight for a clue to a clue.)
Scenario 3:
The door is trapped.
If the PCs have declared ahead of time they're being cautious I'll ask for an investigation to find the trap. In no way does that mean they grab the contact-poison covered door handle. I may ask for checks now and then even on untrapped doors, depends on the game and mood I'm trying to set.
If the PCs are moving at a normal pace, I'll probably use passive investigation possibly with an increased DC.
If the PCs are moving quickly (i.e. bravely retreating from the ancient red dragon they just pissed off), they probably won't get a check.
If the door is not trapped but they still want to investigate it for traps, I let them. The response will be "the door does not appear to be trapped".
Going with contact-poison handle on a door for the setup. I still don't know the overall scenario, but here are some hints that could have been dropped
- On an earlier, similar door, you let them notice the contact poison automatically
- There is a not-so-fresh corpse in front of the door, with a discoloured hand and froth coming out of its mouth. Looks like it died in agony.
If this is an important/significant door, where they are likely to be cautious, the hints could have come earlier:
- The players previously found a small "lab" table with a recipe for contact poison, and some ingredients, including something with a distinct smell (vinegar? ammonia? feces? purple worm slime?)
- At the door, if somebody says they want to inspect the door, let them catch a faint whiff of vinegar.
Scenario 4:
Disarming a trap.
Most of the time I'll just call for a check. Occasionally I'll make it more difficult and ask for details because it requires multiple actions or something that's potentially risky from the perspective of the PC.
There's too much unsaid here for me, but here are some variants:
- If finding the trap was the challenge, then I'd let disarming be automatic if they propose anything remotely reasonable sounding. ("Can I wash the poison off?" "Sure.")
- If finding the trap is easy and disarming the trap was supposed to be the challenge, then I would have used a clue similar to the poison scenario.
- If there's time pressure...maybe they're being chased by something...then I'd use the roll, or multiple rolls, to determine how long it takes. Maybe combat would even start, and there's the rogue, still making an attempt on each turn (and wishing he had taken the Thief sub-class).
- I might even use a straight-up Thieves' Tools roll with some kind of consequences. "If you just try to avoid the scything blades you'll make a saving throw. If you disarm and succeed you succeed, but if you fail you'll have to make the same saving throw at disadvantage."
Again, the theme is trade-offs. If there's no trade-off, then why roll?
This is what I find boring:
Player: "I'll check to see if the door is trapped."
DM: "Roll Investigation."
Player: "17"
DM: "You find contact poison on the doorknob."
Player: "I'll try to disarm it."
DM: "Roll Thieves' Tools"
Player: "8"
DM: "You blow it. Make a save versus poison..."