They only way to prove it has 'something else' it to breed it back to it's offspring. Or, it's offspring together. I would breed it to a normal, pick any strange looking offspring, and either breed them together or back to dad. If you only get yellowbellies & ivories, it's a yellow belly. If something different comes out, then you have yourself a hidden gene.

For example: someone in africa finds a light yellow snake (first pastel). Says, huh this is different. Breeds it to a normal, and has more light yellow babies. That's a good thing. Breeds the babies back to the parent, or each other. Low and behold, supers! The first pastel has been discovered.