Varies by species. I knew the last 2 in the middle of each picture because they have color and they are peacock/hap type. Female peacocks and haps are very drab in color and actually have hardly any color to them. Just very grey.
The blue stripped and yellow stripped fish are Kenyi. Male and female are different in color in adult hood. They are a Mbuna, which can be tough to sex because a lot of species show no difference inbetween the male and female.
The small fish in the upper left of the last photo is a peacock, though i don't know the sex, its still young. Also, female peacocks and haps will not have very pointy dorsal and anal fins, more rounded.
Thats a great background, is it homemade?