When a Tilapia fingerling has reached a weight of one ounce, it can normally be sexed by looking at the genital paella. The genital papilla is located just behind the anus of the fish.
Male tilapia has a single opening at the genital paella, and females have two openings. If you find it hard to examine the genital paella of the fingerlings, try placing a drop of dye, such a food coloring or methylene blue, on the genial region. The color will normally make it easier to distinguish the openings.