The best way to determine that is by using common sense. If it sounds like it wont work then it wont. Though, most of the spells on this site seem to be rather flaky. The best option is to write and cast your own spells, as you have better odds of success in that manner.
Ask yourself if the amount of time it will take makes any sense.
In the case of something like a banishing, that's pretty instant. A job-hunting spell on the other hand... is one just supposed to fly in through the window? That'll probably take at least a week and is pretty doomed to failure if you don't send out your resume.