[Thamnophis]

My female T. sirtalis sirtalis has some red on her sides, but that is only visible when she has eaten.

I think a serious problem with the finding of which subspecies you have is that there are may intergrades "on the market" between different subspecies. The genepool has been "polluted".
That is a good reason to try and keep the species and subspecies as pure as possible.

On the other side... in the wild there are also intergrades between subspecies. And in the wild there are also T. s. sirtalis with red on there sides.