Newworld hit the nail on the head about knowing what they are. Unless you obtained your fish from a reputable source, you'll never know for sure what they are, and are most likely not a pure strain.
There has been a lot of genetic recombining with convicts lately, on the one hand I dont like it, on the other hand the results are so nice I actually brought a little marble convict home from the lfs the other day. I never thought Id ever keep a convict but this one was a no brainer and fits beautifully in my tank.