There are many ways to reveal if someone is a witch, but the most correct one is descibed in the movie Monty Python and the holy grail.
- There are ways of telling whether she is a witch.
- Are there? What are they? Tell us. - Do they hurt?
- Tell me, what do you do with witches?
- Burn them!
- And what do you burn, apart from witches?
- More witches! - Wood!
- So why do witches burn?
- 'Cause they're made of wood? - Good!
- How do we tell if she is made of wood? - Build a bridge out of her.
- But can you not also make bridges out of stone?
- Oh, yeah.
- Does wood sink in water?
- No, it floats. - Throw her into the pond!
- What also floats in water?
- Bread. - Apples.
- Very small rocks. - Cider! Great gravy.
- Cherries. Mud. - Churches.
- Lead. - A duck!
- Exactly.
- So, logically--
- If she weighs the same as a duck...
- she's made of wood.
- And therefore?
- A witch!
Full scene (on youtube)