Hi Lee,
It is funny, I had a new customer buy a Specter from me today and he had a similar question about what to look for. I will post most of the answer I sent to him here.
The main things that all Specters have are striped tails, a stripe coming out of the head, and speckling on the sides. The belly is not one of the main things that determine them. That said most of them do sort of look like Yellow Belly's. Here are some pictures of a Yellow Belly and a Specter belly shot. The first shot is of a Specter. Note the belly has speckling even in the center of the belly (Yellow Belly’s do not) and that it is kind of random. The next picture is of a Yellow Belly. The speckling is much more intense and also note the amount of flames coming up from the belly. Specters do not have flames like that.
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As far as your question about figuring out if it was produced from a Pastel Yellow Belly or a Yellow Belly, why would it matter? As long as one of the parents was a Superstripe you can only have Specters and Yellow Belly offspring. Now on the other hand if one of the parents was just a plain Specter than there is a possibility that a mistake could be made. I do not have that problem since I only have Superstripe Males to breed to my females. LOL
I hope this helps.