Between 1644 and 1647 Matthew Hopkins and John Stearne travelled through Essex, Suffolk, Norfolk and neighbouring counties offering, for payment, to identify witches. In about three years they produced the largest concentration of witch prosecutions in English history.
The timing is not incidental. England was in civil war. The assize circuits that normally brought experienced central judges into the counties had been disrupted, and local magistrates were acting with less supervision than usual. Into that gap walked two men with a service to sell and a technique that always found something.
The technique had two parts. The first was the search: a suspect was stripped and examined for a mark said to be insensible to pain, where a familiar spirit had fed. On an elderly body, moles, warts, scars and skin tags are not scarce, and a searcher who expects to find one will. The second was watching: the suspect was kept awake, often seated on a stool, sometimes for days, while observers waited for a familiar to appear. Prolonged sleep deprivation produces confusion, suggestibility and hallucination, and eventually the watchers would see something, or the suspect would.
Neither method can produce a negative result. That is the whole of it. Hopkins did not need to be a cynic or a believer, and historians argue about which he was; the procedure returned the answer he was paid for either way.