Test of guilt from days of Europe's yore (Days of our Lives?), for the love of god--and fire and water
Toss suspect witch into a river(or some such puddle one supposes). If she be innocent, she'll drown duly, a witch alone uniquely equipped to contrive the dark power to resist drowning, naturally. In which[witch?] case--surviving drowning-- proof that it can only be demon/stration of said supernatural strength, now equitably and safely to be snuffed-out by burning her to death. Slowly but surely, at the stake.