My Itchy has never really been sexed by anyone experienced, and I'm not very good at it. But I'm fairly certain he's a girl. We just still call him a "him" because he's been called that his entire life, and no sense changing it now. Plus, his name is Ichabod Crane. Mrs. Ichabod Crane doesn't have the same ring to it.
He doesn't do anything characteristic of either sex, so I'm pretty sure your best bet is like the person above me said, to post a picture of its vent so more experienced users can give you a hand. But honestly, Itch used to head bob and arm-wave when he was at the pet store (and I visited him MULTIPLE times before I was ready to get him), but now he does nothing.