I used to when I first joined. I used to assume that a lot of the users who had female anime/gaming avatars to be...well female. SO when I quoted someone and agreed with "her" and get a reply back that said "I'm a guy lol" or something like that. Now, before I quote someone, I check their profile and make sure that they are either a boy or a girl and since a lot of the users here are male with the female anime/gaming avatars, I usually know if they are a man but still check just in case I don't mess up.
Edit-Also, when they use the Emoticon like the "XD, >_<,

or ^^" ones for example. I'd assume they were females since a lot of my female friends on Skype use them. Thanks to
this user who explained what they are, I still kind of assume but not so much.