I really think that detecting illusions is a heavily underused mechanic in ttrpgs.
So try this. You make a dungeon. Everything in the dungeon is positioned and behaves as if there are walls. In reality, there are no walls. There are incredibly high level illusions if walls. The doors are real, the floor is real, all the items in the dungeon are real. Some of the doors are even locked. But there's no walls.
And the trick is, unless a player specifically says they touch a wall or they lean up against a wall or anything like that, you say nothin to give them even a hint that the walls aren't real. Because they're going to assume that there are walls, even if you don't have miniatures, or a VTT map, or you don't describe them. It's unreasonable to poke around at every aspect of your surroundings to see if they're real or not.
With perceptive skill alone, a player requires an absurdly high check. 35 or something along those lines.
So there are really only four ways to discover the walls aren't really there.
1) player touches the wall and realizes that it isn't there.
2) player uses magic to detect or see through an illusion. I'm considering extrasensory ability in this category as well, but sensory still has to be mostly intentional because they have to have a reason to distrust their eyes.
3) skill check. They try to look around them to find something. They might notice a slight improbable repeating perfection to the illusion.
4) they beat the dungeon and the illusion fades, revealing how silly they've been this whole time.