But you do not know if the person you are asking is a liar or a truth teller, so you would have to ask the question again to confirm the answer
If you're speaking to the truth teller, then he knows that if you ask him in a moment where the city is, he'll point towards it. He's a truth teller, so he tells you the truth and points towards the city.
If he's the liar, then he knows that if you ask him in a moment which way was the way to the city, he'll point away from it. He's a liar, so he lies about lying next time and points towards it. (The thought process in his head is "OK, so if I was aked in a moment which is the way to the city, I'd point away from it. I have to lie about that, so I need to point towards the city")
Either way, the person ends up pointing towards the city in response to one question.
I think.
Vin