I have no idea how to distinguish the two either, and I cannot find any information online about how to tell them apart. I suppose there's usually little need to identify Callorhinchus species morphologically as the three known species do not overlap in range at all, so location is typically sufficient to tell them apart. Obviously that's not very useful in in aquarium though!
Hm, looking at other images from this facility labelled as C. capensis, it seems that species lacks the dark blotchy markings present in C. milii. So I think it's safe to say this is the latter.
