They can often be found:
1) Spamming the Net with Bieber propaganda
2) Changing a completely unrelated topic of conversation to Justin Bieber
3) Reacting to any criticism of "Biebz" with pure hatred. While normal, rational fans of Justin Bieber have a sense of humor, Biebtards do not. Be cautious, they are contagious.
Woman: Yeah, but I barely read it because somebody spammed comments about how they love Justin Bieber and want to have his babies...
Man: ...What a Fucking Biebtard! :O