Unfortunately, fish are not like mammals. Whereas most mammals have fairly distinctive sex organs and gender traits, the same is not true of tropical fish.
Depending on the species, determining the sex can be fairly easy or is can be near impossible. Some of the more difficult fish to sex include freshwater angelfish, catfish, and gouramis.
Other fish, such as the Betta (Japanese Fighting Fish), are easy to separate by gender. Males have long, flowing fins and bright, brilliant colors, while the females are drabber and have shorter, almost stubby looking, fins.
If you are interested in breeding fish, for most species the best way is to purchase a half-dozen or more young fish of the same species and raise them together. When they mature, they will pair off, and then you can try to breed them and raise the fry (babies).
Before buying any fish, however, it is best to do research to make sure you have the proper set-up (including proper tank size, proper PH, proper water hardnes) for the fish you wish to keep. Within this research, you can find out the best way to sex and breed your favorite fish. Different species have different needs and have different traits for trying to determine the gender.
Betta fish, Aquarium Fish, Tropical Fish