For those who didn't bother to go to the link above, the answer is to ask a gaurd...
"If I asked the other guard which door would lead me to safety, what would he say?"
If you asked the liar, he would say the false one because the honest one would tell the truth
If you asked the honest one, he would say the false one because that is what the liar would say.
So you would go to the oppisite door.