There is no one "fool-proof" rule that will allow sexing of any fish. Each species is different. Learn about your specific species of interest. These are just some of the guidelines that one looks for when sexing a fish. Most adult fish in breeding condition can be sexed with some exceptions like certain plecostomus, loaches, etc. In most of those cases, this is because the species have rarely been bred in captivity.

Size and Shape
Most fish can be sexed by their size and shape. In some species, females are larger (examples = most danios, most cories, white cloud mountain minnows). In other species, males are larger (examples = many cichlids, rosy red and fathead minnows, rosy barbs). And in some species, the sexes are the same size. Like I said, there is no rule. Prior to spawning, most female fish are noticeably more rotund (fatter). Their ovipositors may enlarge. This i a ube from which the eggs flow at the fish's vent. Males often have a more unusual shape. They may have more adornments or bumps.

Colors and Finnage
In many species, males are more brilliantly colored and/or have longer fins. Examples include most labyrinth fish (bettas, gouramis, paradise fish), most livebearers (guppies, mollies, platies), rosy barbs, and white cloud mountain minnows. There are a few species where the female is the "better looking."

Tubercles
In some species, like goldfish, males develop tubercles (also spelled tubercules) which are white spots or bumps on their gill plates and/or pectoral fins. Sometimes, a female can also have a few tubercles or a male may lack them.

Behavior
Males often chase females. In a few species, the female is the aggressor. In species where fry receive care, males often become aggressive and/or make spawning sites or nests. This all depends on the species. Sometimes a fish will not behave as its sex would dictate. For example, it is known that two female angelfish may "spawn" and produce infertile eggs. Also, female zebra danios may become aggressive and chase other fish around.

"Fool-Proof" Method
Okay, there is one way to know a fish's sex for sure. If you witness eggs coming out of a fish's vent, she is female. If you witness milt (sperm), he is male. Oh, but what about hermaphroditic saltwater fish? Okay, so they can produce both! I told you that there is no "fool-proof" method!