[novia]

Casually discuss logic puzzles and probe to see if they understand the concept of a paradox. Talk about the odds of something happening and see if they can think in terms of probabilities beyond 50/50.

I've found myself as a mathematician that conversations just flow with other people who have a math background in ways that they don't with other people. It's probably due to a conversational deficit on my part.

As others have mentioned, something really important is mental flexibility, the ability to look at new evidence and adjust your opinion. You can find out if people have this ability in a casual conversation, but it would have to be a long casual conversation.