well, good question.
In fact, I know at least two answers but due my not good English I say that one which may be easy translated (=

So, I come to one of them and showing on the one of doors, ask:
"what will say other man if I ask him about that this door is safe exit?".
If he answers "he will say NO, it is not" then this is right door. If he answers "he will say YES, it is" then I select another door.

Regards,
Oleg