Overall body color has nothing to do with gender, in my
experience. There are only two ways to be positive. One is
the laying of an egg, the other is to have a DNA test done.
However, there is one visual hint that has proven to be pretty
accurate but only applies to a mature bird and only works on
the Red Tails (Congos). Look at the underside of the red tail
feathers. More often than not, a female will have a slight
grey edging on the red feathers. The males do not. Also, some
say there is a slight difference in the shape of their heads
but to use that hint, even if it is accurate, one would have to
have two of proven gender to use as a comparison.