Instructions
Look at the gourami's coloration. As an adult, the male pearl gourami displays a redness to his chest or "throat" region. By contrast, the female's whitish chest is less distinct from the rest of her head.
Consider the shape and proportions of the fish's fins. The male's dorsal fin --- the large ribbed one on the back --- grows to be long, pointed and flowing, and his anal fin, opposite the dorsal on the gourami's underside, displays extensions. The female's fins are squatter and round-edged by comparison.
Identify a male gourami if the fish engages in mouth-to-mouth combat with another of the same species. These territorial disputes are common to many labyrinth fishes, and may occur in a home-aquarium setting if there is not adequate space for more than one male.
Watch for nest-building behavior. In pearl gouramis, the male takes charge of assembling a bubble nest --- a surface offspring-rearing structure common to gouramis --- within vegetation once the female displays signs of pregnancy. After spawning, the male will collect the floating, fertilized eggs into the nest and tend it closely.
Look to see which parent gourami acts aggressively toward the other after egg-laying, if you missed the nest-building and actual spawning. The male will defend the nest even against the female, so that should differentiate the genders fairly obviously.
How to Tell the Male From the Female Pearl Gourami
Pearl gouramis are large "labyrinth fishes" --- named for a specialized breathing organ --- native to swamps and backwaters of Southeast Asia. Often growing to 3 or 4 inches in length, these are some of the larger gouramis regularly kept in aquariums, but their size belies a relatively docile disposition; they are compatible with many other types of fishes, as long as they are not bullied or harassed by more aggressive tank-mates. Defined by unique color and the labyrinth-fish form, adult pearl gouramis can reveal their gender through both physical and behavioral clues. Immature gouramis may have not yet developed sexually dimorphic characteristics and therefore are often more indistinguishable.