Can we all take a step back here and honestly evaluate how often this kind of thing actually happens? Cause I don’t know about y’all, but I can’t think of a single time I’ve been asked “is the door locked?” or told “I check to see if the door is locked.” Most of the time, the people I’ve played with just say, “I open the door” or, “we go through the west door” and if it’s locked I tell them so. Honestly, once players at my table get used to the fact that I handle knowledge checks passively and the answer to “can I make a (whatever) check?” is always “tell me what you’re trying to do and how, I’ll let you know if you need to make a check to do that” I don’t really get a lot of questions, except to ask for clarification about my descriptions.
This was my middle point about doing things
when it matters.
When my players say "We go through the left door." if the left door is trapped with dangerous magic that as soon as they touch it it will happen, we can likely assume that what the players
did not mean is "We pick the door with the trap in order to get ourselves killed." This is where it's a good time for the DM to "give away" that taking this action without any investigative action may be dangerous.
I think that is "fair" play. The players are clearly attempting to
not 10ft-pole the room, so the DM should be willing to "tip their hand" when it may be a good time for the party to actually put that 10ft pole to use. Otherwise you end up with:
DM: The door explodes when you open it. You all take *dice roll* 55 points of fire damage.
Player1-5 at the same time: That's like double my health man!
DM: you shoulda checked the door.
Player1-5 at the same time: we'd encountered no trapped doors, we had no reason to think
this one would be any different!
DM: Too bad so sad.
I mean there are pretty easy ways to be a jerk DM, and this is one of them. If the DM
knows that a room, a door, a place, a weapon is dangerous, there should be tells to that end for the players to pick up on. "The door radiates an intense heat when you get close to it." (It's trapped!) "The chest seems strangely clean within this dusty library." (It's a mimic!) "You hear a familiar voice on the other side of the door." (It's your mother!)