Depending on age and individual personality, habits may not always work. I have seen some very mellow boys, and some very aggressive hens (especially if they have been kept in a hen section until now). Also, most pigeons will stand their ground, regardless of sex, when it comes to defending a box or perch from other pairs or birds.
Pigeons will pair up on their own, but if for whatever reason you want certain birds together, just put the pair in a separate cage away from the others. Provide a nest box or bowl, and once you see them flirting or sitting in the box/bowl together, you can put them back with the others. Flirting would be preening each other and 'kissing'. You'll also notice some courtship dancing from the male (which they may do anyways just to show off to the ladies) and acceptance from the hen (versus trying to get away from him).
Other than how they act, I determine sex by their pelvic bones. At the vent you'll feel two bones come together in a 'V'. If they are touching or very close together and won't budge, it's a cockbird. If there is a space (usually large enough to fit a finger between...although I'm a girl so if you have big hands, may not apply, haha) and the bones are easy to move (don't push on them too hard, just gentle), it's a hen. Young hens that haven't laid yet can be very tight, so this kind of sexing works best for birds 5-6 months old and up. Some birds have also been bred to have tight vents so...they can be a bit deceiving. I've usually only found that in racing pigeons though, because that's one of the traits some people believe are beneficial. With kings, this method shouldn't give you any problems.
One more thing to keep in mind, if they are new birds, it will take them a while to settle down and show interest in each other.
