They are used in means of Divination to determine whether or not an enchantment has been placed upon a person.  You will need a pot of water and a handful of Oak apples (galls).  The exact number of Oak apples you need varies by region, but the most common number is three.  Place the Oak apples into the pot of water and slide the pot under the bed of the person you suspect is enchanted.  You must observe absolute silence during this practice.  If the Oak apples float the person is free of enchantments, if the Oak Apples sink the person has been enchanted.