The perception on this question will differ greatly depending on whether you believe a witch to be born or whether you believe an individual chooses to become a witch. The truth of course is more complicated than either answer would suggest. It is certainly my view that somebody who chooses to follow the path of the witch will have some inclination toward the craft before he/she decides to incorporate it into their life. This doesn't necessarily imply an established birth line or the ability to trace your ancestors back to finding evidence of actual witchcraft but it does mean that it is unlikely anybody ever leaps out of bed, throws on a magic robe and becomes a witch in the blink of an eye. There is undoubtedly some background preparation going on before any commitment is made.
In my view a person doesn't become a witch until they have articulated their intention to practise as a witch to themselves. I do believe that certain people have natural abilities and a predisposition to a craft path but unless these abilities are acted upon and worked with, to my mind that person isn't a witch. I would define a witch by what she does, not who she is. So it is not so much a case as knowing that you are a witch as much as it is a case of acknowledging the potential within yourself and recognising your genuine interest in following a craft path. So the things I would be looking for would be - interest in witchcraft, an ability to work magic or a belief in the properties of magic, an understanding of what following a craft path involves and an intention to make a commitment (to yourself, not necessarily to a deity) with regard to practising the craft. If all these elements were in place and a person chose to start incorporating witchcraft into their lives then I would certainly consider that person a witch.
To summarise my answer to your question - you don't know you're a witch, you know you have a desire to follow a path that will lead to you identifying as a witch.
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