Cual o Que???
When do you use cuál over qué????
It can be complicated as Marianne said. After a lot of practice you begin to feel which word is correct, cuál or qué. Generally though, if there is a finite number of possibilities, cuál is going to be used, as in which. If there is an unknown number of possiblities for an answer, qué may be more appropriate or correct. There's more to it, and there's always exceptions.
This question has been asked a few times before -- it's a complicated subject.
Here's a good thread on that.