This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
Two men stand at a fork in the road. One fork leads to Someplaceorother; the
other fork leads to Nowheresville. One of these people always answers the
truth to any yes/no question which is asked of him. The other always lies
when asked any yes/no question. By asking one yes/no question, can you
determine the road to Someplaceorother?
The fact that there are two is a red herring - you only need one of
either type. You ask him the following question: "If I were to ask
you if the left fork leads to Someplaceorother, would you say 'yes'?"
If the person asked is a truthteller, he will answer "yes" if the left
fork leads to Someplaceorother, and "no" otherwise. But so will the
liar. So, either way, go left is the answer is "yes", and right otherwise.
It is possible, of course, that the liars are malicious, and they will tell
the truth if they figure out that you are trying to trick them.