Prison Guard

You're in a prison with two guards & two doors. The guards decide to play a game, and offer you freedom through one door or death through the other. You are allowed exactly one question to exactly one guard. You are also told that one of the guards, you don't know which one, will answer the question truthfully & the other is guaranteed to answer with a lie, and that the only possible answers are Yes or No. You have one question. How do you get out alive?