The same way I know leprechauns don't exist.
You are the one positively asserting the existence of real witchcraft. It is up to you to prove this is so, not up to me to prove a negative.
That's incorrect, because there is no empirical evidence for leprechauns, while there is empirical evidence for the existence of witchcraft and other supernatural and preternatural phenomena, unless you rule it out a priori.
Here is an example of a phenomenon that has no scientific explanation, although it is not an example of witchcraft.
We have accounts from Catholic exorcists of exorcees levitating, speaking backwards, speaking in dead languages and revealing facts of which they could not have knowledge. There are several logically possible reasons for dismissing their accounts, none of which are reasonable.
They could be lying, but why would they lie? What would they have to gain? If lying, all of the eyewitnesses would have had to have lied too. Why would they all conspire to lie? What would they gain? Why haven't any recanted?
The events could be natural phenomena, but there is no reasonable natural explanation for these phenomena.
In a positive sense, we can know with moral certainty that witchcraft is possible.
The Bible provides accounts of witchcraft > We can trust the Bible to be the Word of God > We can trust the Bible to be the Word of God because the Catholic Church teaches this > We can trust that the Catholic Church is Christ's Church because it fulfills a 2000 year old prophecy. > The Bible tells us that Jesus built a Church, and predicted that the "gates of hell" would not overcome it. > The Church's ecclesiastical structure has been in place for 2000 years with an unbroken line of papal succession, and the Church possesses a 2000 year old body of non-contradictory dogma, comparable to no other earthly institution. > We know that the Bible is the most reliable ancient document of its era since 500 copies of it exist which date to before the year 500 A.D., and all copies agree substantially.