You don't. Belief in witchcraft is heathenism. Anyone engaging in demonstrable altering of reality is in fact performing a miracle, and those can only come from God.
Ye olde witch trials were almost entirely performative and were levied at people who would have overall been pariahs anyway; the Inquisition (pick one, really) demanded hard proof of witchcraft, but their accusers were never able to dredge anything conclusive up; it was the locals who were doing this, not any broader authority. Malleus Malificarium is quite literally some of the horniest German monk schizoposting committed to paper.
However, if you are, in fact, dealing with someone you believe to be a witch, you just have to weigh her to see if she weighs the same as a duck. If she does, she's a witch.