Good question Vortex, you'll have to use your own criteria for that and in general you must read the effect of the spell and consider that it isn't very powerful, for example for destruction spells those of low level would be ones that affect a singlr target on touch. Spells that buff the character are usually low level too. In illusion chameleon, which makes you invisible when you are not moving and shadow, which makes you invisible in dark places, are low level while invisibility is high

Best way: just PM me if you have severe doubts