In D&D there's the notion of spotting secret doors, traps, etc. passively. How does one handle that in dungeon world? Discern Reality appears to be an active roll on the players part. How do players just happen to notice the pit trap because the thief catches a glimpse of a wire hanging off the trap out of the corner of his eye?
[RPG] n equivalent of D&D’s passive checks in Dungeon World
dungeon-world
Best Answer
As you mentioned in the comment you probably need to shift your mindset a bit. I would think about what you are trying to achieve in the narrative with your trap. And then do one of two things:
If you are trying to use the trap as a way to convey that the location is dangerous. (This could make a good "Point to a looming threat" dungeon move) Then I would just tell the thief
The thief can then choose what to do. You don't need to roll for a perception check, and this is totally OK. Regardless of the outcome the party is now aware that the location is trapped, and you will have achieved your goal.
If you are trying to make the place actually dangerous (perhaps you've already shown it is dangerous some other way) I would let the thief look for traps without dropping any hints. If the thief looks for the trap (triggering Trap Expert) you can follow the move to its logical conclusion, otherwise you can spring the trap on them. Either way you have succeeded in making the place dangerous.
As a general principle for Dungeon World, never roll dice unless you've triggered a move that tells you to. If you find yourself in a scenario where you want to roll dice but no move has been triggered, take a moment to figure out what you are trying to achieve and how you can make it happen by following the GM's rules and making a fitting move.