Determining the gender of a betta fish is generally quite easy, but sometimes they look similar enough to make a decision challenging. Taking the following characteristics into account should help you determine the gender of your betta. Keep in mind that juvenile fish may not display sexual differences. Ideally, compare mature specimens of the same species and color, and compare multiple factors rather than using a single trait to determine gender.
Often males are more vividly colored than females; however, color alone is not a definitive predictor of gender. Males generally display more vibrant colors than females, but females can be quite colorful, too.
Female bettas will display vertical stripes on their body when they are ready to mate, while males do not.
Generally, females are a bit shorter and more wide-bodied than male bettas. Males tend to have more elongated bodies that are slightly flatter, side-to-side.
Male bettas have much longer fins, sometimes as much as three or four times the length of the females' fins. While some varieties of bettas have males sporting short caudal (tail) fins, in most varieties the females have shorter caudal fins and the males have longer fins. The ventral fins of the male are noticeably longer and thicker than those of the female.
Mature females display an “egg spot” between the ventral and anal fins. This is actually the ovipositor, which is used to lay the eggs. Males rarely show an egg spot.
Bettas have a membrane beneath the gill plate cover called the opercular membrane. This membrane appears as a “beard” and is displayed when the fish flares its gill plates. Males have a much larger beard, so large that often it is visible even when the male is not flaring. Females also have a beard, but it is much smaller and not visible when the female isn’t flaring.
Male and Female Flaring
When bettas flare, the differences between the sexes become more apparent. Males display a large beard, while females have a much smaller, less pronounced beard. Females may also assume a head-down posture when flaring, a posture that males do not exhibit.
Male bettas are nicknamed Siamese fighting fish for a reason: They're very aggressive with one another and with female bettas, violent enough to lower their expected lifespans. This is why you should never have more than one male betta in a tank. It's also recommended not to combine male and female bettas in an aquarium, except for mating.
Female bettas aren't nearly the fighters that males are, but they can be aggressive toward one another and to other fish. Aggression between females can be very stressful, especially if there are only two females and one tends to bully the other. For this reason, it is recommended that if you are keeping more than one, you should keep at least five females in the same aquarium so that aggressive behavior is more diffused and not directed toward the same individual.
Generally, only male bettas blow a bubble nest. This is a nest made of saliva bubbles that the fish creates on the surface of the water to protect the eggs during breeding. Again this is not absolute, as occasionally a female will blow a bubble nest. However, those instances are fairly rare. Males create bubble nests in preparation for breeding with a female and will make nests, even if they have no mate in the tank.
Agonistic Behavior in Betta splendens. University of Miami Department of Biology
Betta Fish Anatomy - Plus Male And Female Differences. Bettafish.org