Before they spawn, their little dance is rather unmistakable. It's definitely not just swimming together. The female develops a very intense purple belly. She swims around the male curving her body and showing off her belly. The male also develops more intense colours, and extends his fins whenever the female is close. They poke each other's sides with their mouth, and they go and visit possible spawning spots. Sometimes for several days. Then eventually the female disappears in a cave/coconut shell/flower pot, whatever you can provide them for spawning (they like to spawn on cave ceilings).
Another sign, by the way, is that they usually become pretty nasty with other kribs in the tank. Sometimes with other fish too, I read (my krib tank is a species tank so I wouldn't know about that).
It's been pretty obvious for the 2 pairs I have observed. If you haven't seen any of this behaviour but they are just swimming calmly together in their regular colours, they are probably not just about to get busy...