Male opaline gouramis actually create a nest out of bubbles and plant bits for their eggs. To breed this fish and observe this behavior, you must first get a male/female pair, since multiple males may fight in a group. While the differences between males and females in this species are not obvious, there are several important tell-tales.
Though this difference may seem subtle at first, males and females of this species have differently shaped bodies. In general, males have slimmer, more angular bodies than the females, which are usually more rounded. You can usually see this difference best when viewing the side of the fish. When conditioned for breeding, this becomes more apparent, since females tend to swell with eggs.
The dorsal, or back fin, is probably the simplest way to tell a male opaline gourami from a female. In males of the species, this fin tends to be larger and longer. Additionally, the shape is different. Female opaline gouramis have a shorter, more rounded dorsal fin. In males, the back of the dorsal fin forms a point, which "points" backwards relative to the fish.
The size of the fish can help you differentiate between their sexes. In general, males will grow slightly larger than females. This is called sexual dimorphism. However, you can only count on this characteristic when you know that the fish are of the same age and have had the same level of care. An older or better-fed fish will usually grow larger regardless of sex.
Males of the species tend to get much more aggressive than their female counterparts. Males may fight among themselves, particular when they're conditioned for breeding. Additionally, only male opaline gouramis will build bubble nests. If you see a fish blowing a raft of bubbles in the aquarium, it is most likely a male.