From a distance, male and female Blue-Footed Boobies look virtually identical:
But if you get close, you can tell them apart by their eyes.
Female boobies will appear to have larger pupils than males. The pupils are not actually any bigger, but female boobies have a ring of dark pigment around their pupils which the males lack, giving the illusion that their pupils are larger. In the image above, the female is on the right.