The giant gourami Osphronemus goramy Lacepède (1801) is one of the main freshwater commodities of economic importance in Indonesia. This species has been produced in small-scale farms for decades. Although giant gourami aquaculture has grown exponentially during the last 15 years, there are still limitations in the availability of fry, in part due to difficulties in sexing broodfish, which leads to non-optimal sex-ratios for breeding. In this study, morphological and behavioral criteria for sex identification based on the Indonesian National Standard and a field survey were assessed on >400 giant gourami broodfish using a random forest algorithm. The actual sex of the fish was confirmed using a urogenital cannulation technique. This analysis demonstrated that, for the so-called “black” phenotype fish, observations of the hump on the forehead, thickening of the lower jaw, and the pigmentation on the pectoral fin are highly reliable for sexing (about 95% success). However, pectoral fin pigmentation was not useful for sexing the so-called “white” phenotype fish. This study also revealed that the criteria often used by fish farmers do not improve sexing success. Based on these findings, recommendations are made for optimizing the sexing of mature giant gourami fish and for selecting preadults as future broodfish.