Its difficalt for the best breeders to know what other morphs are in a Bel. The Lesser mojave is quite predictable, but the black pastel would only be proved with breeding out in my opinion. As for any other hidden genetics, its possible as the Bel's whiteness hides other morphs.
One way that you can try and see other morph patterns it to shine a UV light on it. That sometimes shows other hidden Morph patterns.
Breeding out to a normal is the best way though.
New to ball pythons but not breeding snakes, so other members please contradict freely or advise further.