Most hobbyists, whether because they plan to breed their fish or because they simply like to know, want to be able to tell whether their fish are male or female at some point. Determining the gender of your fish can be a frustrating task, and in some species is actually impossible to do visually. Because of this, the best way to determine your fish's gender is to research the individual species. However, there are a few handy tips to help determine whether your fish is male or female that can be used in many different species.
Make sure that your fish are sexually mature. This will vary depending on species, but waiting until the fish are at least a few months old is generally a good start, as most fish do not display obvious differences between the genders prior to this point.
Look at the fish's coloration to note differences between males and females. Many male fish are considerably brighter or bolder in color than their female counterparts, a trait used to attract mates and discourage rivals. Dwarf gouramis are an excellent example: males are often boldly colored in orange or blue, while females are generally a more demure brown.
Compare the size of the fish to one another. This may be easier in a school of fish, where you are better able to estimate a median size by which to judge, but in many fish the female is larger than the male, or more rounded in the belly region.
Observe the area around the anal fin area on the fish. The anal fin is the fin on the underside of the fish, just behind the belly, and this area is where many fish display reproductive parts. In many species, such as guppies and some cichlids, the anal fins on males are either much larger, or much longer than the female's. Guppies, for instance, posses a "gonopodium," which is an elongated anal fin used for mating. Sharks, meanwhile, posses "claspers" located in this area, which are used to hold on to females during mating.
Watch for breeding behaviors such as mating and egg laying. If all else fails, this may be the only sure-fire way to determine the gender of your fish.