Dah clock one (look at a clock, look away, look again - has the tien changed?), the nose one (pinch your nose shut - can you still breathe through it?) and the finer one (count yah fingers - do you have the right amount?).
Those are my three favourite ones (with the clock one being the most successful for me).
EDIT: Oh, and - The BIG reality check topic - Part II 
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