I've heard this so many times, and have been told THE answer. But I never understood it, and I can't quite remember THE answer.
Isn't it somewhere along the lines of, if I asked the other twin which way is the right one, what would he say?
That's correct. Imagine THE rigth answer is "left". If you ask THE trufhful one what THE liar would say, he will answer "right", since he knows he will be lying. If you ask THE liar what THE truthful one would say, he will lie and say "right" also. So afterwards you know you will have to pick THE other road.