Unless they're the type to flaunt the fact that they're a female, or they speak in vent, chances are pretty good that you'd never be able to tell if a player is male or female. There have been times where I mistook a guy's mannerisms for female ones (I honestly thought he was a she until he spoke in vent), where on the flipside I've often been mistaken for a male due to my mannerisms when I type (or speak, though I have a distinctly female voice). So while many females have a distinct give-away for their gender (how they speak even in text, which is often but not always more verbose), for the most part you won't know until you know, which is to say you'll know when or if they want you to know.