the females will get really pink in their abdomen when they're ready to mate or full of eggs. the male will get red around his face and bottom half when defensive/aggressive.
its very easy to sex kribs though, the male are noticeably larger and longer. http://www.spheniscus.ch/datafish/up...20pulcher.html
male on left, female on right.
cichlids are all a funny bunch. while dwarf cichlids are considered "passive" they can be very very aggressive with each other. i've a breeding pair and the female will get very aggressive with the male when there is a failed clutch.
you really have too many cichlids in that tank as well. the two rams and two kribs is asking for disaster. probably better off replacing the kribs with more rams as they'll form a harem (while kribs form pairs)