BEL with extras conundrum

I'm sorry if this is posted somewhere else, I was having a tough time getting the search terms correct. I have a question about BEL combos, as in how would you know if you had one? If you have some sort of BEL, and an extra gene that's not a pattern changer (pastel, hypo, axanthic, or whatever) how would you be able to tell it's actually there assuming the parents are not both supers or visuals (if recessive)? I get that you could possibly see a pattern change under uv lighting, but how do you determine the presence of other genes in a plain white snake?

It's all about who you get it from and if you trust them. The Only way to be 100% would be if you bred a super form... say a super pastel lesser x lesser would result any bel produced being a pastel bel and so on for other morphs. Other than that... It's all about who you trust and even then they could get something like that wrong. Just have to breed it out.

It would be something I breed myself in a project I'm contemplating, so I can (mostly) trust the breeders competence :D. But there's a chance of BELs from the pairing, which would make it rather difficult to determine if they were actually axanthic, which is the aim.

I can't and all of my BELs this year came from combinations like Pastave X Lesser or Cinny Lesser X Mojave. When I sell them, I tell the buyer that there is a possibility of another gene, but there is no guarantee. The only way to find out for sure is to prove it out through breeding.

I've not had a BEL to try it on....but I've heard putting one under a black light exposes otherwise hidden patterns. Even if that works I am not sure it would be a true give away.