This puzzle can be posed in many ways. Friend Timm Wheeler showed it to me in about this form:
You come to a fork in the road. One path leads to Nirvana; the other, to Hell, but you do not know which leads where. A being stands at each path of the fork. One always tells lies; the other always tells the truth, but you do not know who tells what. You must ask one question of one of the beings to determine which path leads to Nirvana.
For the purposes of testing, let the Left path lead to Nirvana.
Which road leads to Nirvana?
Truthful: The left
Liar: The right
This one is a no-brainer. You don't know which road goes where, and which guy might be lying to you. If you just pick one of them and ask, you have no way of knowing whether it's true.
Does the left fork lead to Nirvana?
T: Yes
L: No
Different question, same problem.
Were I to ask the other, "Which fork leads to Nirvana?", which would he say?
T: Right
L: Right
If you ask Truthful which fork leads to Nirvana, he will say Left. If you ask Liar, he will say Right. If you ask Truthful which fork Liar would say leads to Nirvana, he will truthfully tell you what Liar would say; therefore, he will say Right. If you ask Liar which fork Truthful would say leads to Nirvana, he will lie about the answer Truthful would give; therefore, he will also say Right. The path to Nirvana is always the opposite of the answer you receive to this question. We now have a good answer. This sounds a lot like the common solution of asking "Which way would the other being tell me to go?" but see my opinion on that below.
Here are alternate solutions that illustrate the principle of asking one being what the other would say, in which we can see how one will truthfully report the lie, and the other will lie about the truth.
Were I to ask the other, "Does the left fork lead to Nirvana?", what would he say?
T: No.
L: No.
Were I to ask the other, "Does the right fork lead to Nirvana?", what would he say?
T: Yes.
L: Yes.
There is a popular answer to this puzzle that I see many people use. It's easy to see when someone has Googled the solution instead of working it out because they will quickly produce this:
Which way would the other being tell me to go?
This is widely considered to be the correct answer, and understandably so. After all, if you ask the truthful one which way the lying one would tell you to go, he will point to the path to Hell, and if you ask that of the lying one, he will also indicate the path to Hell. Right? Maybe not.
That answer presupposes that either of the beings have your best interest at heart, or that they are partial in any way to your fate. A logic puzzle such as this must not be compromised by such assumptions which have not been explicitly stated. The solution that I have reached above is the only way I have found to certainly and logically determine which path leads to Nirvana, and which leads to Hell. Amazingly, I have been accused (by aforementioned friend Timm Wheeler) of adding unstated information to the puzzle in pointing out this logical flaw!