I would say everyone who practices magick believes in it because if they didn't they would not be doing magick but some sort of play acting I would imagine. Without belief there would probably no clear inent or purpose and so the magick would not work.
That is clearly not the best explanation but I'm sure someone else will further what I've said as I am far too tired.
people who cast, and not to see if it works, do believe in it to some degree. at first i feared it backfiring, so i was afraid to cast, but once i learned about it, i'm not as concerned. myself, certain spells i have a hard time believing in so i don't cast them. i only cast when i desperately need help, otherwise i leave it alone.