"If I ask someone from the other town if your town is Truthville, will they say yes?"
If the man is from Truthville, someone from the other town would have to lie, and would say "no". Our guy has to tell the truth therefore the answer would be "no".
If the man is from Liarville, someone from the other town would have to tell the truth, and would say "no". But our guy would have to lie about it, and the answer would be "yes".
So if he answers "no" to that question, he is from Truthville. If "yes", he is from Liarville and you want to go the other way.