First, you read. Then you read some more. You read the good stuff, you read the fluffy stuff, you read the stuff that you adamantly disagree with. Then you take some time off to assimilate it all, and really figure out what speaks to you. Then you ask yourself questions. Then you go back and read more, based on the answers to those questions. Then you look for other people to ask the same questions of. Then you figure out that half those people don't know what they are talking about. Then you read more, scour the internet and ask more questions. After all that, you read more. Once you figure out what speaks to you, where your interest lay, what (to you) is not right or applicable, then you start delving deeper.
You'll know once it feels right, and it may take a day, a week, a month or decades. I disagree that you wake up one day and decide that you're a witch. It's not that easy. You can become one, for certain, and you will know when you are. But its not like picking out a jacket.