Find a corner of the room you are in where two walls and the ceiling meet (it's like three triangles forming a sort of Y). If you look at it right it can cause an optical illusion where it looks like a corner on the outside of a cube rather than the inside. You might not see it straight away, but it's there. If you're in a dream there is absolutely no chance of something strange NOT happening... when awake you can just sort of see it for a bit before it's a corner in the ceiling again.

I appreciate that this might be a little trickier than most, but make it work for you and it's a mind blowing reality check.