The idea of a reverse Turing Test ("prove to me you are a machine") has been rattling around for a while but AFAIK nobody's really come up with a good one
Maybe asking how it reacts to a turtle on it's back in the desert? Then asking about it's mother?
Solve a bunch of math problems really fast? They don't have to be complex, as long as they're completed far quicker than a person typing could manage.