One is the classic shape-shifting witch losing a paw -- hand -- which usually has a ring, which definitely does help on the identifying business. You can get a good spooky tale about someone being hassled by a cat in a lonely place.
Or overhearing something they say, or even where they meet -- now the meeting is a real good chance, if the witches go and brag about what they've done.
I still think "ply them with strong booze" is probably your best bet when catching a witch.
