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 changed.
Looking at one's digital watch (remembering the time), looking away, and looking back. As with the text, the time will probably have changed randomly and radically at the second glance or contain strange letters and characters. (Analog watches do not usually change in dreams, while digital watches have great tendency to do so.)
Flipping a light switch. Light levels rarely change in dreams.
Looking into a mirror; in dreams, reflections from a mirror often appear to be blurred, distorted or incorrect.
Plugging one's nose shut, and attempting to breathe through it, or attempting to breathe underwater. It is usually possible to breathe while doing this because the tester is not actually plugging their nose in real life.
Gripping and stretching a finger. In a dream, body image can become distorted, and pulling a finger can elongate it. Also, the number of fingers can shift when stared at.
Jumping into the air. Gravity is often distorted in a dream state and floating or flying may occur.
Looking around and seeing everything blurred, as if underwater.
Imagining being sealed inside an invisible force field. Most of the time, the dream will radically change to a state in which one is sealed inside something (commonly a glass orb) and/or lost in sound, vision or mind.
Being able to move through solid objects like walls.
Putting one's finger through the palm of the other hand.
Closing one eye and looking at one's nose. The dreamer may not see their nose as everyday details that usually go unnoticed in waking life are often absent during a dream.
Acting as if one has "super powers". In a dream the "super power" attempted will occur and help the dreamer realize that they are dreaming.
Pinching one's self, or having someone else pinch you. "Pinch me, I think I'm dreaming". Perception of a pinch may be altered in a dream state.
Action The dreamer, another dream character, or a thing does something unusual or impossible in waking life, such being able to fly, or noticing photographs in a magazine or newspaper becoming three-dimensional with full movement.
Context The place or situation in the dream is strange, and includes fictional characters or places.
Form The dreamer, another character, or an object changes shape, is oddly formed, or transforms. This may include the presence of unusual clothing or hair, or a third person view of the dreamer.
Awareness A peculiar thought, a strong emotion, an unusual sensation, or an altered perception. In some cases when moving one's head from side to side, one may notice a strange stuttering or 'strobing' of the image.
Cohesion Sometimes the dreamer may seem to teleport to another location in a dream, without a noticeable transition.