Many (not all, but many) female/non-colored-up Protomelas can be identified between species because of the differences in melanin patterns. There are also other obvious species where you can tell them apart (Nimbochromis and other predators, C. moori, Exochromis).
On peacocks, a few have distinguishing characteristics (red jakes, crested dwarfs, Ngara stuartgranti), but most appear like the picture you posted. I have even seen great variation within a collection point (light and dark-colored Bengas for example).
That is why I never recommend keeping female peacocks in mixed tanks.