It's easier to tell which are females at first. Females can make the male call AND the two syllable female call. You can do an online search to hear the difference. Once you hear it, you'll know for sure. The female makes the two-syllable call that (to me) sounds like "pot-RACK" - to others it sounds like "buck-WHEAT" Males cannot make the two-syllable call, so you have to watch them if they're free-ranging.
PeepsCA told me a sure-fire way to tell them apart by their calls - separate them out, one at a time, and within a few minutes, the female will start calling. I did this with my four, and it worked like a charm, although for some separating them can be difficult. I herded each guinea from inside the pen, into their coop and closed them up.
BUT - some females are very quiet and it can take some time, LOL. With another guinea I had, I had to wait for an egg. She would only make the female call when there was perceived danger. But generally, they'll "give it up" if you can separate them.