An Easy But Smart Logic Puzzle

Puzzle Description:
There’s a prison and it has two gates, one is towards outside, the other is towards a dead end. There’re 2 guards guarding this 2 gates respectively, and it’s interesting that one of them always tell the truth, the other tell the lies. If you’re the only prisoner in this prison, you’re allowed to ask only one question to one of the 2 guards to get to know which gate is towards outside, which isn’t. So, what’d you ask?

    Well, it's not sure which guard guards which gate. So think carefully and give it a shot. It's a really smart puzzle, also, if you google it, you'll find it an ancient logic puzzle. But! Don't do that before you start any thinking. :-).