Well, there are a few ways of "testing" your success. You could actually test your success by doing very small scale things. Admittedly, it's easier to do with something like divination where you can set it up so that someone else knows the answer but you don't or something similar.
From what I can tell though most magic is more subtle than trying to find a correct or incorrect answer (which is a 50/50 chance anyway). Magic may help to shift the balance in your favour just slightly. Sometimes it's enough for things to go your way, other times it doesn't change anything. I'd say that if your intentions are manifesting, you're doing something right (magically or not) and there's no need to alter much. Either the balance was already in your favour or you managed to tip it.
(I would like to clarify that when I say "balance," I am using it purely as a metaphor. I'm really referring to the balance of statistics and the probabilities of intentions manifesting or not. Balance is only used because it sort of sums it neatly)