Ask conditional question.
The two door problem. Try one of the following questions directed to either man.
"If I asked your companion, what door would he tell me to use?" Liar will lie about the truth tellers's response. Truth teller will tell truth about liar's response. Either will indicate the wrong door: Take the other door.
"If I asked you: Which door would you say is the good one?"
Truth teller will obviously give correct answer. Liar must think as follows: "If asked, I would haven given him a bad answer. I must lie about what my answer would have been, so I have to point to good door." Both men will indicate the good door.