In yours (yes, she's definitely a girl), it's the color pattern that gives it away. Generally the females have distinct blue in the dark border of the hind wing, and light pigmenting in the middle of the border of the front wing. In the males the dark border is usually more nearly solid black, and the blue may be absent entirely (though there is usually a little bit). The dark markings also tend to be wider in females, and the wings just a tad less elongate, giving a slightly blockier appearance to the butterfly. The shape of the abdomen is also a bit different, wider in the middle, and more evenly shaped. When seen from the proper angle, the structures at the tip of the abdomen are different in males and females, but you can't see that in this shot.

She's lovely.