The gossip test:

"Working in the Admiralty, I had several friends among the naval officers. They were interested in science but knew even less about it than I did. One day I noticed that I was telling them, with some enthusiasm, about recent advances in antibiotics - penicillin and such. Only that evening did it occur to me that I myself really knew almost nothing about these topics, apart from what I had read in Penguin Science or some similar periodical. It came to me that I was not really telling them about science. I was gossiping about it. This insight was a revelation to me. I had discovered the gossip test - what you are really interested in is what you gossip about. Without hesitation, I applied it to my recent conversations. Quickly I narrowed down my interests to two main areas: the borderline between the living and the nonliving, and the workings of the brain."

Francis Crick, What Mad Pursuit