Identification
In the season one episode "Witch" a method is demonstrated that, by performing, a witch can be identified by using various ingredients; some of their hair, a little quicksilver (mercury), some aqua fortis (nitric acid), and "eye of newt". The ingredients are heated then applied to the possible-witch. If a spell has been cast in the previous 48 hours, the witch's skin turns blue where the substance comes in contact with it. This was used to discover that "Amy" (actually Catherine) was a witch.
Read more about this topic: Witch (Buffy The Vampire Slayer)