They tend to pair up on their own as they mature. Mine do not always stay with the same mate tho, and sometimes there are trios of birds or one male with a small harem of Hens, lol and they even divide into small sub-flocks that are inspeparable.
It's next to impossible to tell the sex of Pearl Grey keets until the females start making the 2 syllable buck-wheat or come-back call at around 5-6 weeks. Some do it sooner, some do it later.
Most of my Hens develop a bustle on their rump area as they mature (16wks old+) and have a low profile to their posture (unless they are screaming at something/someone of course). Then of course if the bird lays an egg, it's obvious.
The males only make the one syllable chi chi chi alarm call and have a different posture, they are more upright and they tend to act like they are constantly on guard, bully and chasing anything anyone that catches their eye, especially at feeding time and during breeding season. When they chase or charge something their wings are tilted and raised over their back a little bit (looking tough). I've seen Hens mimicking this behavior tho, so don't go solely on that tip.
Some Guineas you may never know the sex of until they lay an egg or you see them in the act of mating