Apart from the spotting, the most important and obvious difference is that a fingerling / juvi fila will grow 3x-5x faster than a capa, ~6"+ a month. Or so they, who had filas, say.
So in 1 month there should be no mistake of telling true, healthy capa from true, healthy fila. As for not-yet-described species, that can easily be true. Capa was split from fila only recently by Dr. Lundberg and taxonomists guess that less than half of all Amazon fish species have been described to date!
IIRC, someone even spoke of hybrids between the brachies. Fugupuff had posted pics of some pretty amazing, different-looking juruense, for one. So that also throws a wrench in our wheels.
If you read the link I provided and see all the plentiful photos of capa vs fila juvies by Fugupuff and Co, then you'd know as much as I know on this issue. This thread is the only or rather the most helpful and reliable (as in people sharing the knowledge are trusted experts) source for the purpose, ime and imho.
I am not aware of authoritative claims that mouth shape, eye position or size, or the "viciousness" of "face" can be used as distinguishing traits. Fugupuff spoke only of smaller spots and the spots not occurring below lateral line, and longer (if intact) maxillary barbels as distinguishing traits.
Furthermore, when Necrocanis, who collaborates with Dr. Lundberg, reached out to him for ID advice, Dr. Lundberg got back with what he stated in his revision paper: only tooth patches can be used in reliable distinguishing of capa from fila. See the paper for that, please.