I've only been learning Spanish a month but I think "qué" means "what" and "cuál" means something closer to "which" in English. Often in English you can say "what" to mean either "what" or "which." In many other languages, you need to actually distinguish the difference. By this logic, you should use cuál/which when you are asking a question where there are available or known options to choose from. Qué/what should really be reserved for when there aren't any known options to choose from. I could be completely wrong, but this is the understanding I have. In Dutch they do a similar thing where "wat" means "what" and "welke" means "which" and you have to actually differetiate between the two.