Board Thread:Show-Related Questions and Answers/@comment-27180081-20161021134432/@comment-26116863-20161025014336

Well, she is a woman who has magic, so...

But so do fairies, and other female magical creatures.

I guess they have to either identify or be identified as a witch.

Monty Python solution:

''The only way to tell is to see if she balances out with a duck. Witches burn. What burns apart from witches? Wood! So witches are made out of wood. Wood floats. So do ducks. So if she is made out of wood, she will weigh the same as a duck :P''