Board Thread:General Discussion/@comment-1279279-20160330131948/@comment-5558012-20160330175551

You can't determine a user's gender by their username. Typically looking on their profile and profile header is the first step.

As for what to call them, if I don't know their gender I'd call them by their username or IP. Additional mentions in the same conversation they'd be reference to as "they, the user/IP, this person," etc. "It" isn't appropriate for a person.