One way is to ask one of them, "If I were to ask you which path goes to (my destination), which one would you point to?" The truthteller will point you in the right direction, because he would have anyway and would be telling the truth. The liar will point you in the right direction too, because he would have pointed you in the wrong one, but will lie about that. You don't even have to ask twice, since they'll give the same answer.