ANSWER - I'm not entirely sure if you mean how do I know personally or how can the universal "you" know if they are a witch. So I'll answer both ways and hopefully I'll get it right!
I know personally that I am a witch because I regularly and consciously use magic to influence my life and the lives of others. Its a case of "I do therefore I am." I practise Witchcraft, I utilise magic, I consciously control what happens in my life with my craft practises so I can be fairly confident I'm a witch.
The above applies to other people of course, I've said it before many times - If you practise witchcraft you're a witch. But what I think you mean is how do you know you're a witch before you've consciously started to practise the craft. There can be many different signs that you are a natural witch (and I differentiate natural from intentionally practising without making a value judgement. In my experience whether you are born into the craft, learn the craft, or have a natural ability to practise the craft it is no indication that you are any more/less a witch than anyone else.)
I think you can start to consider the possibility that you may be a natural witch if you either start to demonstrate abilities outside the normal five senses or you start to develop an awareness of beyond the scope of your own life. This can be exciting powers like psychic ability (yet again, not all witches are psychic) or something as simple as a philosophical and naturally enquiring mind. You will have an interest in controlling your own life and a desire to do so. Very few witches are passive, we are by nature proactive souls who seek to influence our own lives. A natural witch will have the idea that there is something more she seeks from her life, something outside convention, something she may find hard to express but an understanding that there is more to life than can be obtained within the confines of the lives so many are happy to lead. She will know she is a witch because one day she will wake up and decide that it isn't enough any more, that she wants more. And the determination will actualise her power until she takes the control inherent in a witch and changes her life so radically that there is no going back. She opens doors, both in her life and in her mind and she strides forward with the confidence of knowing that life as a witch and from here on in is on her terms.
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