For sexing kribs, look out for the following:
Dorsal fin - on a male it should be longer, stretching out sometimes past the base of the tail. It will come to a definite point. On a female it will be blunt and rounded.
Belly - a female will have a rounded belly. A male shouldn't have a pronounced belly, and he should be longer and more 'torpedo' shaped.
Tail - a male's tail looks like the head of a spade.
Colouring sometimes can help - females will often have a very brightly coloured belly, ranging from pink, through red and purple. HOWEVER, males can get his colouring too, so make sure to check for the other things!
Nip over to the african cichlid section, there are a few krib topics there and you will be able to find some pics of males and females.