Dwarf gouramis (Trichogaster chuna) have interesting breeding behavior. They will actually construct a nest made out of bubbles for their young, and tend to them until they hatch. In order to breed these fish, you must first make sure you have a male-female pair.
Males and females with wildtype coloration are easy to distinguish, though different populations have slightly different color patterns and these fish can change color to some degree. Males generally have a black or blue coloration on the face, throat and abdomen, which tends to remain relatively constant. Females have a dark horizontal stripe along each side, though this can be faint in some individuals. Additionally, the females' stripes can become more or less pronounced as the fish changes color.
You can also tell these fish apart by their size. Females are larger than males. An adult male dwarf gourami will reach a standard length of 1.75 inches. A female will max out at 2.2 inches. "Standard length" is the length of the fish from the tip of the nose to the base of the tail. If you use length to determine gender, remember that these measurements are for full-grown fish of the same age who have had the same level of care.
You can also tell males and females apart by their behavior. Males act more aggressive than females. This species is rather harmless, so the aggression rarely has any serious consequences, but males will chase other fish around. Additionally, only the males build bubble nests. If you see one of your fish crafting a raft of bubbles at the water's surface, that fish is a male. Additionally, only the male will tend to the young.
Aquarium hobbyists have used selective breeding to produce a number of variations of this species. Unfortunately, this can make the differences in coloration between males and females less pronounced. However, the methods of comparing their behavior and size still apply.