I go by avatar/sig and sometimes the name and determine that way. Though, it's not always the most accurate, as I've made a few mistakes in the past. I thought Levia was a girl because I associated the name as feminine. Jayren (Rainecloud) uses the female avatar/sig themes quite alot, and he's male. =p
So, going by this method is not always the best. If I'm really unsure, I just check the profile.
Lazy bastard.