First, ask yourself why you need to cast a spell? Do you really need a spell for a certain purpose where natural means aren't possible, or just to see if they work.
If the purpose is truly valid, then know the spell will work, not just believe.
From a book I once read(can't remember the name), it states,
To believe is to have doubts.
To know is to perceive directly from the senses and mind.
And study up upon the Hermatic Quatranary...To Know,To Will,To Dare...and To keep Silent. Basically the four sides of magic.
Hope this helps,