
Turn the pupa (chrysalis) over. Now, look at the abdomen (pointy part of the pupa or the part that was hanging down if your pupa was on a stick). You may need a magnifying lens if you can't see very well!
IF you spot a vertical line (see picture below) then you have a female Giant Swallowtail. If no line is there, then you have a male. The blue marking (oval) indicates what the line looks like.
Click on the picture for an enlarged view.

Isn't that easy? Simpler than you probably thought, right?