Their ovipositors may enlarge. This is a tube from which the eggs flow at the fish's vent. Males often have a more unusual shape. They may have more adornments or bumps. Sometimes, a female can also have a few tubercles or a male may lack them.
Males often chase females.
In a few species, the female is the aggressor. This all depends on the species. Sometimes a fish will not behave as its sex would dictate. Also, female zebra danios may become aggressive and chase other fish around. Okay, there is one way to know a fish's sex for sure.