Most of the problem in identifying live Mesonauta specimens is that the 5 or 6 species (I think we're up to 6...) are largely separated by drainage (location) and colour pattern - from an aquarist point of view. I don't have the references in front of me at the moment, but if I remember right, it is the fright, or black, markings that are used. The short answer is that unless you know specifically where your wild-caught fish came from, it's unlikely you'll ever get a definitive identification of the beasts in your aquarium. Unfortunately, farmed-rasied specimens and those circulating in the trade could be hybrids of a couple of Mesonauta species or at least have been "domesticated" long enough for the colour pattern to be no longer useful in arriving at a conclusive ID.
If you can, find out where your fish came from and we may be able to offer a little more. If you can't get/find that info, enjoy the fish anyway - their breeding/brooding behaviour alone makes more than worthwhile!
Best of luck with them.