I am in the process of updating a phoenix article i wrote years ago with new information.
to keep things simply however, heres a *general* rule of thumb.
no color at all on fins/stripes - fake insignius, 6-8"
orange top fin? probably insignius or one of the other species. I have not seen an exceptionally large FF with orange top fin. probably insignius
spotted body = insignius or sub species, 10" average -> max size
no spots on body, no orange top fin, probably true taeniurus , known to hit 24"