It'd probably be similar to beefield's. Present a problem with a clear success condition and see how resourceful they are in solving it. A failure would be solving the wrong problem, making excuses (or "the problem is not solvable"), confidently talking in abstractions that don't concretely pertain to the problem, or failure to ask a question/say "I don't know".

I don't know that there's a technical question or problem I could ask to make a call on whether or not they know their stuff.