Normally the best way to see wheither its a guy/girl IRL playing a female character ingame is by looking at how they styled their character.

A guy would normally never bother with glamouring or even customizing his character in the creation screen, they would just pick at random and play.

Although, there are some guys that make their characters look like total slu**, half naked etc and well its obvious then. (some girls do this too sadly but not the majority).

A girl IRL would try to make their character as cute as possible, with dresses, makeup, hair styled perfectly to match and colour matching.

But ya thats my idea on it xD This is how I work it out and it hasn't failed me yet lol.