Sexing bluegills:
Males have deeper bodies and much more color than females.
The females generally have vertical bars on them and the males develop
a deep orange breast area when they are ready to spawn. Males will
make nests in the substrate. One trick that we found was to put a line of
plants or other stuff in the tank partially blocking off about a foot of the
end of a tank. A male would then take over this end area and start nesting.
Nest making will be obvious -- the male sweeps material out of a circular
patch with his fins, particularly his tail fin.