[xzcat]

That's the joke/trick, it's basically begging the question. You can out a cheater by asking a question of the form:

Which is true, (A and A->B) or (A and A->C)?

A cheater will fret over whether B or C is true, while someone who did the work will probably immediately identify that A is false and that the question is misleadingly worded.

[2zcon]

It takes a very low false-positive rate before most of the people you're catching are just nervous, not cheaters.