Which is Which?

I found this "puzzle" from a manga I'm reading: Majin Tantei Nougami Neuro; volume 14, ch 118: Gate.

Here it goes!

Situation: You're in front of two gates: Gate to the Earth and Gate to the Demon World.

Before each stands a guardian, one who only tells the truth and one who only speaks lies. You are allowed to ask only one question to only one of the guardians. Well, which guardian would you ask what question to return to Earth?

So the question: Which is the gate to the Earth and which is the gate to the demon world?

Mission: Choose the gate to Earth (to live, of course!).

Reminder: You do not know which guardian speaks the truth or only lies. You do not know which gate is which. You can only ask ONE question to ONE guardian.

Hint: Positive(+) times(x) negatives(-) & negative(-) times(x) positive(+) = negative(-).

Try to find out the answer! When you find it, leave in on the jottings below together with your reasons as well! Happy trying!