Depends on the species, but determined from an opening near the fish's anus.
can you tell if a fish is a boy or girl by the tail
no one
you can tell by the dots or spots on the boy fish and on the girl is just a gold fish colres
the boy has a ball sack
no one
put a mirror in front of the tank if the fish bloats out its gills its a boy.
Look under its skirt
By the size, it's patterns/colours I BELIEVE TAHT THE GIRL TROPICAL FISH HAS EITHER A LONGER TAIL OR IS A LIGHTER COLOR THEN THE BOY TROPICAL FISH
A person can tell if a koi fish is a boy or girl by looking at their fins. A female will have fan-shaped fins with very little color.
Just look between its fins, silly!
fish boy
A girl Betta is dull and not as pretty as the male. But, a boy Betta is colorful and pretty to attract the female mate.