look at the eye mask or eye bar.
a burundi type frontosa will have a eye bar that looks exactly like a stripe, more or less up and down. they will also be more silver with black stripes.
a bismark will have more of a eye mask instead of a stripe/bar.
they will be more blue and black instead of silver and black.
they will be a bit more stocky too.
do a google image search for each, and look for the eye stripe.