Brown
The male is bigger and it has a bigger wingspan than the female.
The mature male has brighter colours and larger flowing fins than the female.
Mallard Ducks, Pheasants.
Nyala
The male of the species has brighter colours and is a much larger fish than the female
Most species have no sexual dimorphism, meaning male and female look pretty much the same. At most, they'd have different colours, the female being a bit more drab and the male more brightly coloured with striking colours on his throat or belly.
The male is larger and has much larger fins and much showier, brighter colours than the female who is quite a drab little fish.
Female guppies are larger and plainer. The male is distinctive for his long, flowing, colourful tail and generally brilliant colours.
No a black sheep can be either male or female, there are no coat colours that determine the sex of sheep unlike cats.
Many female birds are colored duller than the male, because she does most of the incubation, and the cryptic coloration helps her blend in to the environment.
In the wild, male guppies have flashy colours to show off to and attract a mate. The females are virtually colourless to avoid the notice of predators. In aquariums we have increased the colour of both the male and female, but the tendency of the male to be more colourful remains.
If your Betta is developing the long fins, bright colours and male temperament, then you can safely assume it is a late developing male fish and was incorrectly sexed as a youngster.