Here is an excellent FAQ about lucid dreaming. It says:

Carry some text with you or wear a digital watch throughout the day. To do a reality test, read the words or the numbers on the watch. Then, look away and look back, observing the letters or numbers to see if they change. Try to make them change while watching them. Research shows that text changes 75% of the time it is re-read once and changes 95% it is re-read twice. If the characters do change, or are not normal, or do not make sense, then you are most probably dreaming. Enjoy! If the characters are normal, stable, and sensible, then you probably aren't dreaming. Go on to step 2.

(I know, there's lots of new age bullshit at that link, and in lucid dreaming generally. But no matter how much you cringe at the word "dreamsign", lucid dreaming is a very useful tool if you can spot and dismiss the garbage.)