Matured male kenyi's usually intense aggression towards each other to the extent that it may be hard to keep two fully matured males in the same tank. As juvies they're more tolerable of each other but the aggression will develop.
With maturing two juvie males, one may increasingly try to dominate the other to the extent that it may focus on that activity which will eventually ramp up to full blown aggression. Established mbuna's have a formidable 'home team advantage' so I would expect your pre-existing evident male to be the dominant by default. If the new fish is some how able to counter and reverse roles that would be a clue that it is a male.
From what I've seen any minor 'sparring' between juvies usually indicates the participants are of the same gender.
Males can harass and display aggression towards females as well (to the extent that it's usually recommended to keep kenyi's in harems so any one female doesn't receive all of the male's attention). Male's may try to attract the female by 'displaying', showing some interest in her, before following up with aggression if she's not receptive.
Some times the above activity can be subtle and because cichlids can be driven by a lot of individuality, I only suggest it as a clue or small part of the gender solving puzzle.
