Nice thread, it might be worth stickying, depending on how it goes

Tests I use(d):
- Hold your breath with your nose, but keep trying to breath. Check if you can still.
- Count your fingers
- Look in a mirror and touch your nose (sometimes hard to find one)
- Try to recall what you ate last, and if it makes sense.

Others:
- Throw a ball in the air, and catch it
- Flick light switches
- Try to fly (problem is if you can't normally fly...)
- Ask DC's whether you're dreaming. Their response isn't normally the usual 'Are you insane?' or 'huh?'