These creatures are obviously more dangerous than standard wild animals as they possess the physical powers of a beast and the mind of a human. There are a number of ways to determine whether your friend Bob is actually the drooling monster that ate your cat last weekend. First of all, there are often cosmetic differences between a regular human and a lycanthrope. Lycanthropes very often have a physical resemblance to the animals they become. Of course, this is open to a lot of interpretation and if you start shooting silver bullets into everyone who looks like a dog you aren't going to be very popular at the holiday party.
Lycanthropes are often very full or very sore after a night of carousing as an animal.
But other activities can also induce these symptoms so you can't be sure. You can also check for wounds on the person if you noticed them on the animal. While these things heal very quickly, if you shoot off a wolf's paw and notice that Bob is missing a hand the next day, well, its a clue. Jewelry on an animal is also a hint. Of course, the most surefire way to ascertain what you are dealing with is monitor the individual.