In cases where I can't really tell, I just play it safe and use "they"...

...Which is grammatically correct, and even if it somehow isn't, it soon WILL be, what with everyone using it and the fact that language changes over time.

@Clitheroe: It's been correct for a long time. Even when people used "he". People just never worried about it because back then women actually were ignored. It just never adjusted after we started realizing we're operating at 49% efficiency.