That's assuming you know which one is truthful and which one is the liar. I'm not sure but I think the riddle should imply that you don't know, which in that case I would ask, "Would you say yes if I said is this the right way to go?" or something along those lines.

Lets say the way the truth teller stands in front of is the correct way, if you ask him the question then he would say yes, the liar on the other hand would say no; as he would lie and say yes, but then lie about that answer so end up with no, if you get what I mean. Two wrongs making a right

.