Helman-Ziegler test

The Helman-Ziegler test was, according to the Twelfth Doctor, a reliable way to assess whether people are 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)