Instructions
Examine the color of the bluegill. Male bluegills tend to be a lot brighter than females, particularly in the the breeding season. If the fish you have caught exhibits bright coloration, then it is most likely a male.Males also tend to have a definitive spot at the posterior base of the dorsal fin and square, and heavily pigmented opercula lobes (the area covering the gills); in the female, the opercula lobe tends to be rounded and less pigmented.
Check the fish's abdomen. During the breeding season, female bluegills tend to have a fuller, rounder abdomen than the male. If the abdomen is swollen in any way, then you are probably dealing with a female.
Examine the fishes urogenital opening. A mature male bluegill's urogenital opening usually terminates in a small, funnel-shaped pore. The area around the opening tends to be darkly pigmented. The female's urogenital opening tends to resemble a small, swollen ring.