You hang out in front of your aquarium and see which restroom they use. lol
Not really, and of course I'm only joking. Many fish can be sexed by the size of the fish, live bearers have a modified anal fin, some cichlids have "egg spots on their anal fin, and some fish, such as loaches it seems that the female is plumper and larger around than the male. Sexual morphisms in many fish are,s sometimes very hard to dissiminate. I still have trouble after having been in this hobby for 45 years.