If they throw seed packets and yell, "Fireball! Fireball!" They're probably a LARPer. If they have green skin and orc teeth... probably a LARPer.

I was more a World of Darkness larper than outdoor fantasy stuff. It is, admittedly hard to tell which of the people dressed in black and wearing vampire fangs at the goth club are larpers and which are just goths (both, luckily, fall into the category "mostly harmless").