You could ask either of them what the other man would say. If you asked the the one who was telling the truth you would get the right answer and if you asked the one who was lying he would lie and give you same answer as the first, therefore by asking either of the men what the other would say you would get the incorrect answer. For example if you didn't know whether to travel north or south and you wanted to go south and you asked the honest man what the liar would say he would tell you north then if you asked the liar what the honest man would say he would lie and say north. So whichever one you ask you should do the opposite of what they say. Hope that helps.