I've talked to a few breeders and basically there are great examples of fires, then there are "fires" which look normal but are still het lucy. A friend of a friend ended up producing a BlkEL from what he thought was a pretty normal, didn't have a clear belly, hardly a head spot, still dropped a couple BlkEL.

It's like yellow bellies, just sometimes you don't know for sure and have to prove it out. Honestly if it doesn't scream fire to you, I wouldn't bother because the offspring are most likely going to be hard to tell and sell. I would just buy something that is no doubt a fire, even if the less appealing ones are cheaper.