Brain Teaser

Solve this: A room has two doors. Behind one is a million dollars; behind the other is a fire-breathing dragon. Two guys are in the room one of whom always tells the truth while the other always lies. They both know what lies behind the doors, and they both know about how the other guy either always lies or always tells the truth. You have only one opportunity to ask only one of the guys one question that will indicate to you which door to open to get at the million dollars. What question do you ask, and of whom?