It is just going to take experience. Subtle differences in shape of the clown and the look of the eyes.
Wild caught clowns are easy to tell which species. Tank raise with minor deformilies and due to people like me, who interbred them, we cannot always tell.
Regarding Black and white clown, Mature Ocellaris are all black and white. Mature ultra Onyx Percula will always have bright orange face and slight amount of orange elsewhere on the at least some of the fins. I have never seen an Percula with complete black and white.