Reality Checks?

I've read here that you should do reality checks often during the day. I'm not really sure what a reality check is though, other than what seems obvious by the name; checking to see if you're dreaming. What sort of reality checks are there? How do you do one?

Ahem

Ah, thank you. Guess I should have looked for that...

No problem, just use the search function next time. ;)

Thanks.