Determining the sex of fish seems to be a very popular question. This is not always as easy or reliable as one might hope; nothwithstanding, there are several techniques for determining a fish's gender.
If your fish are dimorphic (i.e., males and females have distinct coloring patterns), the task will be rather easy. All you need to do is find pictures of both the male and female for your species, and then simply compare. Sometimes fish that are dimorphic have very subtle differences and distinguishing them can be difficult, and may not always be apparent. In this case, solving the mystery of gender for these fish should be addressed as if they were monomorphic (i.e., males and females look similar).
Determining the gender of monomorphic species requires a process commonly called "venting."This involves removing the fish (with wet hands) and examining their dorsal side, looking for the genital pore between their anus and anal fin. As these twopictures demonstrate, the more anterior pore is the anus, while the second pore is the genital papilla, or vent. The genital papilla (also called an ovipositor for females) is the pore through which the fish will excrete either eggs or milt.
Generally speaking, females will have a larger genital papilla than their male counterparts. This larger size is needed to accommodate the passage of eggs. This difference is more difficult to distinguish for substrate egg layers (e.g., Neolamprologus spp.) because their eggs are smaller than those of female Mouthbrooders.