"There was a notorious Witchfinder in Scotland who undertook by a pin to make an infallible discovery of suspected persons, whether they were witches or not. If, when the pin was run an inch or two into the body of the accused party no blood appeared nor any sense of pain, he declared them to be witches, by means of which no less than three hundred persons were condemned for witchcraft in that country."

I'm not sure this is an infallible method.