I think the most definite tell-tale sign is the head scalation, this pic shows the two. Sebae on the left, natalensis on the right.
image
I think the other variables can be a little more inaccurate, if I'm right in believing. They're the speckling on the chin, the tear drop under the eye and over all colouring.
Hopefully someone with more experience on natalensis may shed some more light.
I'd personally love to own one, but they seem to be like rocking horse :censor:
EDIT: Also, Python natalensis is no longer a sub-species of Python sebae, it was given full species status.