Cape House Snake or Brown House Snake - how do I tell which one I've got?

I recently acquired an adult female house snake – she’s definitely either a Cape (L. capensis) or a Brown (L. fuliginosus) … but I don’t know how to tell which species she is.

I’ve read some things stating that Capes have eye stripes that break at the back of the skull and that Browns have stripes that continue unbroken down the neck – but I’ve also seen photos of a clutch of animals from eggs with good fertility – who show both traits. Ours has broken eye stripes that stop at the base of the neck and do not continue from there, and she has very, very minimal markings generally. Our female, Cinnamon (so called because she looks like the cinnamon phase animals I’ve seen photos of) is a former rescue animal, so we don’t know what locality she came from or whether she herself was captive bred.

Is there a reliable way of distinguishing Cape House snakes from Brown House snakes, other than breeding an unknown-species animal to a member of each of the two species and seeing which one produces viable hatchlings?