Silver. To the heart is most effective - either silver bullets, or a silver knife. Knife's a good way to test and be sure it's not just some dude with a freaky skin condition, too. Just nick 'em with the blade a little, first.
Careful, though. Thing's probably already shed and taken a new form, by now. Could be anybody. Probably better off waiting 'till someone can get there and help track it.