A few different ways. All general of course, because of breeding you could run across a hybrid Mbuna that has different species in it. But generally speaking with pure bloodlines...
-A male saulosi will usually have an/some egg spots on its anal fin.
-Also, male saulosi's bars will typically run the height of the body, while a female kenyi's stripes generally fade out about halfway down.
-Kenyi get larger then saulosi.
-saulosi males will have a solid black line on the top of their dorsal fins as well as solid black pectoral fins.
-A female saulosi is going to pretty much be 1 solid color while a male Kenyi can vary with like light colored fins and such. And again, if they show male kenyi's stripes will fade out about halfway down the body.