The answer is simpler than that, bbarr. You would simply have to ask "If I were to ask you if this is the correct path to take, would your answer be 'no?'"
1) If he is a truth teller, and it were the correct road, he would reply "No, (I would not say no"
2) If he is a truth teller, and it were not the correct road, he would answer truthfully "Yes, (I would not say no)."
3) If he is a liar, and it were the correct road, he would reply "No" to the question posed above (that is, the same as the truth teller). Why? Because if he were asked directly "Is this correct path?" he would say "No (it is not)," lying. However if asked would you answer "no" (the answer to which is yes) he must lie and say that he would not say no (lying about a lie).
4) If he is a liar, and were the incorrect road, he would reply "Yes." Why? Because if he were asked directly "Is this the correct path" he would reply "Yes (it is)," lying. But, again, he must lie about lying; he must say "Yes, (I would say 'no'
" precisely because he wouldn't say "no."
Perhaps this isn't simpler, but I think this is the correct question. Your question would be if there were two natives there "If I asked your buddy here...etc. etc."
Just a thought.