They growl a lot.
Full moon they grow lots of hair!
They howl at the moon.
The same way you can tell if they are a Vampire or a Leprechaun . . . It starts with an absence of rational thought, and an influx of naivety and superstitious nonsense.
You can go ahead and hold up Wolfsbane to them though . . Just for your piece of mind.
You have to have a real good imagination.