"If you were to answer the question, 'Does the right road lead to Hell?' as honestly as you're answering this question, would your answer be 'yes'?"
In order to understand it, let's suppose that the right road leads to Paradise.
If the person you're talking to answers your question honestly, then they would answer the question "Does the right road lead to hell?" honestly as well, and would answer "no." Therefore, their honest answer to your actual question is "no," they wouldn't say yes to the question, "Does the right road lead to hell?"
If the person you're talking to is lying, then they would answer the question, "Does the right road lead to hell?" as yes. HOWEVER, they're going to lie about what their answer to that question would be: they'll tell you that they would answer "no" to that question.
Got it? So ask them the question, rely on a double-negative to cancel out a lie, and trust the answer you get.