The best way to check a dark cat for fleas is to stand them on a white sheet, towel or piece of paper.  Give them a good full body scratch (they normally like this).  If you see what looks like little pieces of charcoal on the white surface, you can assume they have fleas.  These little pieces are dried blood excreted by the fleas.  To make sure you are correct, drop the grains into a little water and watch the water turn red.