Something that is made of wood and floats in water. It can be confirmed they are a witch by seeing if the "witch" weighs the same as a duck, which also floats in water. Since witches are made of wood, which floats on water, the witch would have to float like a duck. Therefore they would weigh the same, making her a witch.
Bedevere: Does wood sink in water?
Peasants: No, no, it floats! It floats! THROW HER INTO THE POND!!!!
Bedevere: What also floats in water?
Peasants: BREAD! APPLES! VERY SMALL ROCKS! CIDER! GREAT GRAVY! CHERRIES! MUD! CHURCHES! CHURCHES! LEAD! LEAD!
Arthur: A DUCK!
Bedevere: Exactly. So logically...
Peasant: If she weighs the same as a duck, she's made of wood.
Bedevere: And therefore?
Peasants: A WITCH!!!!