There are two twins who look EXACTLY alike, one always lies, one always tells th!

There are two twins who look EXACTLY alike, one always lies, one always tells the truth...?

you can only ask ONE of them ONE question. what can you ask so that you can tell which one's which?

Ask one, "If I ask the other one if he's lying, what will he say?"

If you asked the one who always lies, he'll lie and say the other one will say he's lying. If you asked the one who always tells the truth, he'll truthfully say that the other one would tell you he's telling the truth (which would be a lie).