Well, lots of "Discoid" roaches in the hobby are actually hybrids of B.craniifer and B.fusca, and you can easily tell those apart from B.discoidalis because they are larger and have different coloration than true discoids.
However there are also hybrids between B.discoidalis and B.atropos, B.sp "Venezuela" and other closely related species that will be marked as discoids too, and I don't really know how to tell those apart. Ask the person you bought them from where she got hers.