Helman-Ziegler test

The Helman-Ziegler test was, according to the Twelfth Doctor, a reliable way to assess whether people were dreaming or awake. During the test, the people in question were each asked to take a copy of the same book. In the real world, those books would be identical, but as they did not exist in the subjects' memory, they could not be identical in a dream. (TV: Last Christmas)

Clara Oswald also uses this test whilst in a dream state after being captured by the Zygons. In this case she reads a magazine to analyse if the the words make rational sense. After finding many inconsistencies she surmises that she must be dreaming. (TV The Zygon Inversion).