Answer:
According to Adam Goodheart in his article How to Spot a Witch, there are several "foolproof" methods as to how to see whether someone is a witch or not.
1. shave their body to see if they have a Devil's mark or extra nipples
2. pour holy water over them and throw them into a pond; if they float, they are a witch; if they sink, they're holy people
3. catch them near their imps; since it was believed that every witch had a familiar, or an imp, just keep close watch over them and eventually, their imp will appear to prove they are a witch
4. ask them the right questions