These redtails can become large and live quite long, tbh. The grey belly can be tricky to use as a reference and so does the size. For not all specimens will grow up the same max size wether that concerns a female or male. And when an adult male becomes excessive in size, the black on his belly will become grey as well. and when both sexes will become excessive large, the black on their whole body will become less black and becomes more a faded black.
It's easier to tell when both sexes are present and they are of the same age when they're smaller than 4 inches to see the difference between male and female.