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. The Doctor asked Clara Oswald three numbers for the test. She chose 57, 24 and 12. (TV: Last Christmas)

While captured by Truth or Consequences, Clara was stored unconscious in a Zygon pod. Her consciousness manifested as a dream. Clara used a similar test by trying to read a magazine in her dream. It was nothing but gibberish except for words bleeding through from her link with Bonnie. (TV: The Zygon Inversion)