By looking at the closest fin to the tail of the fish you may tell.The males have one and the females have two beside each other.
How do you tell a male and a female fish apart
Depending on the species ,in some the males are very colorful , like in the guppy..or will have much fancier fins like the betta.
You can tell by how beautiful they are brad is hot
Don't know to be honest if you wait they may give birth and the one who gives birth is the female also in mating season the male will form white dots around it gills and fins