Ive heard some male kribs have a faint touch of red on their bellies, but in the two males I have, they dont show it, only the females. Id say the main thing to look for is the dorsal and tail fins though. On the male's dorsal fin there is a line of yellow/orange that extends all the way to the tip, which is also pointed. Allong the top of the tail as well, there is a more noticable line of that color. In the female the dorsal fin is rounded at the end, and the color can almoast reach, but it doesnt.
Be careful to get a pair as well, 2 males or 2 females will try to kill each other, as I found out the hard way.
Also, I dont know if mine were a special case, but in the first pair I finally got, the female was a moody fish, and tried to kill the second male as well. I ended up getting 2 pairs and letting two pair off, and removing the two 'loosing' fish to the community tank. This is in a 10gal however, so with an aquarium twice the size they might learn to get allong as the two in the community tank eventually did.
Theres also two little regular dianos that keep insisting on spawning *every* day in the tiny patch of java moss. The kribs will sometimes have a go at em, but I have yet to see any dammage.
Good luck with your fish.