Betta fish - Fighter
The difference between male and female

A female betta fish-fighter will have a small transparent circle (the egg-bearing tube) on its underside.
This circle resembles a grain of salt, and it is located at the edge of the ventral fin near the fish's head.
This method is considered correct for identifying a female.

The male betta fish-fighter will have large fins.
The male fish can reach a size of 12 cm.
What distinguishes males from females is that they are larger and their colors are brighter.

Compare their body shapes. There are subtle differences in body shape between male and female betta fish (fighters).
Males are usually tall and thin.
Females are usually shorter and fatter.
That's a subtle difference.