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.