I already do these:

Hold your breathe and see if you can breathe.
If you can, you're in a dream.

Look at letters quickly so you can't memorise what it truely says but you see what letters are there.
Look back and see if any letters are changed around.

Look at a clock, turn away then look back again.
Has it changed dramatically? (e.g. 3:50 - 2:10)