The Cauldron of Death

Once upon a time, there was a prisoner trapped in the Cauldron of Death. There were two doors to exit from, one leading to the hungry alligators and the other leading to the actual exit. There were two guards who were keeping vigil of the prisoner. The prisoner, a clever man, bribed them so that they would tell him which door to go through, but he could ask only one question to only one of the guards. One of the guards always lied while the other had never ever said a lie in his whole life - He didn't know which one. The prisoner managed to escape from the clutches of the alligators and the guards. How? What question do you think he asked one of the guards?