How would you go about prosecuting a witch in a world that had functional witchcraft?
A witch makes a little ship of eggshell, and sinks it -- and leagues away, a ship wrecks, and all its crew are lost. A child dies of fever, because hidden somewhere in the village is a poppet that was burned. Witches cook up a cauldron, and a horrific hailstorm tears up the crops and breaks all the tiles on all of the houses. And all three hit the problems that in real life, brought down the witch hunts. How did you prove that it was witchcraft and not natural occurrences? And even if it were overtly magical in origin, how would you track down the culprits?
Worse still if all that witchcraft really was diabolic in origin, so you couldn't call in counter-magic. Divination would be needed at the very least to find out anything. And a heavily forensic magic would be useful. Perhaps something could be whipped up with theurgy, and directly calling on angels or other good supernatural, but that has a long history of being impiety -- which is not exactly what you want to indulge in when facing devils.
Of course, if it were heavily dependent on Stuff, as many historical practices were, that might help. Learned magic for instance -- great thumping books of spells would tell you what spells were cast. And other things might be distinctive.
Might. What fun.