Witch finders are a type of character who appear in the Wardstone Chronicles.
The Quisitor hunts witches, but his victims are usually the falsely accused. Also sees a spook as an enemy of the Church. When he catches someone, he holds a quick trial. There he acts as judge.
Uses swimming to test a suspected witch, throwing her into a deep pond. Innocent if she floats (sometimes drowns!) but guilty if she floats. Also uses a needle to find the Devil's mark, an area on a witch's body that's not supposed to feel pain. It's just cruel torture.
After being stabbed with the pin many times, the poor woman's body becomes numb and doesn't feel the needle go in. If she doesn't cry out, she's guilty and burned at the stake.
(The preceding is taken from the Spook's Bestiary/Tom Ward's journal).
Witch finders and Quisitors alike, are agents of the Church, although some communities take the law into their own hands. Many of the tests that are used do not work, and Spooks do not hold with them.
Witches are seen as rivals to organised religion, and are consequently persecuted.