Guineas don't have much in the way of external sexual dimorphism: there's some slight differences in head shape: the males tend to have blockier heads and larger casques ("helmets"). But these are relatively slight differences. Some "experts" say you can tell by wattle shape, but I found out that half of my "males" I based on that were actually hens.
Anywhere from 6 weeks on the females will start making a two-tone call. They'll do this when they're separated from the flock or when it's springtime and they're looking for a mate (at which point they'll make this noise CONSTANTLY).
All guineas will make a single-tone call when agitated or when the males are trying to call females:
Males will tend to charge other guineas with their head down: this is either to chase off rival males or to see if the hen is willing to mate.
When mated guineas are foraging the male will often sit down while the female forages. They'll also stand guard outside a nesting area while the female lays eggs. If you're near a female guinea in the nest they'll also make some unique sounds in there:
Males are also more "friendly" in that they're usually the ones that will come check you out when you enter the yard. This is the opposite of my chickens where the hens are the ones that some of them will literally try to hop on your lap while the roosters tend to keep their distance.
As to why they're ostracizing another guinea: who knows? It could be the male-to-female ratio is way off (they prefer 1:1), or somebody didn't share a treat with them, or they just made eye contact one day and decided the other had to go. Guineas love to fight each other. In the summer I had two males that would spar and chase each other for hours even though there were enough hens to go around. Now that it's getting colder it's the girls that get into scuffles.