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 Heaven; the other
fork leads to Hell. One of the men always answers the truth to any yes/no
questions asked of him, the other always lies. Can you find a question
that will allow you to determine the road to Heaven?
One method is to point to one of the paths and to ask either of the men
"Would the other man confirm that this was the path to Heaven?" It is the
path to Heaven if and only if the man answers "no".