How do you know if your magic?

Answer 1

everyone is capable of preforming magic, no one individual is magical or not. Magic isn't like Harry Potter, it works subtly and for everyone in different ways
Magic is art which you practice and develop however the level of magic each individual can have access too differs to self belief, concentration etc.

Simply stated, if you are magic or magical, you know. If you don't know, you aren't.

Answer 2

When you cut yourself do little broom sticks come out? (sorry, I couldn't resist).

Seriously though, the idea of heredity in magic doesn't seem to really matter. I've known people who have claimed to be the 4th (5th or 100th) generation of witches/wizards/shaman and couldn't cast their way out of a wet paper bag.

The only way to really know if you have the capacity for magic is to try it. Keep a journal and keep track of everything you try, the conditions you tried it under (were you tired or hungry? Was it the middle of the day or the middle of the night?) and what sort of results you had.

Start with things that have specific conditions and definite time limits. For instance, casting some sort of "I want money" spell is impossible to gauge its effectiveness. You might find a penny in the parking lot and that was the money you cast for. However, something like "I want 15 dollars by monday" is specific and has a time limit and you can very easily say if the spell was a success or not.

Be honest and keep track of ALL your results. You may find that magic just isn't in you.

That being said, there are a variety of groups ranging all across the spectrum that claim they can teach you to be an effective magic user even if its not "in your blood". Some are more successful than others, but its going to be up to you to decide how you want to pursue things at that point.