from Scott Adams' (author of Dilbert) The Joy of Work:

"All you is unexpectedly ask the most direct question you can, then listen for which of the two possible approaches the person uses to respond. For example, you might say, "Did you kill your neighbor?"
Innocent people answer that sort of question by saying, "No, I didn't!" or "Are you &%#$ crazy?"
Guilty people say, "Whatever gave you that idea?" or "There's no evidence of that!" or "Did someone say I did?"
The main difference is that the guilty people are attacking the evidence, not denying the event.