Reality testing (or reality checking) is a common method used by people to determine whether or not they are dreaming. It involves performing an action with results that will be different if the tester is dreaming. By practicing these tests during waking life, one may eventually decide to perform such a test while dreaming, which may fail and let the dreamer realize that they are dreaming.
Common reality tests include:
Looking at one's hands one or more times. Hands generally look distorted, or grow additional fingers in a dream.
Reading some text, looking away from the text, and reading it again - in a dream, the text will probably have c