How do I tell if a beta fish is male or female?

If it’s a big-finned, flashy kinda fish that looks like this (but could be some other color, too, like red or almost purple), it’s a boy.

If it’s a small-finned, less vivid color that looks something like this, it’s a girl. :)

The males are the ones that people usually buy, all pretty and lonely, in those little cups of water. You sort of have to go out of your way to buy a female.

Any elaborate fin-age means male. The females can be relatively colorful, but they have “normal” looking fins.

As the others have said, you more than likely have a male. Most places only order in the males because of how colorful /elaborate their fins are. I know at my store at least you cant get females unless you special order them.

I have a bland looking female in one of my community tanks and is one of the more interesting ones to watch. The other fun watchers are some leftover bait minnows I put in there for shits and giggles. Males tend to be a-holes of the tank especially if any other fish have large fins, or really any kind of fins. When mine died I could notice more harmony in the mix.