From Journey North:

Do you know how to tell male and female monarchs apart?

There are a few subtle differences to help you, and once you have them down, they can be easy to tell apart when a monarch is at rest.

Males have thinner veins and a black spot on the top surface of the hindwing. You can see them here in these photos, but when a monarch is in flight, these differences are more difficult to pick out.

There’s also a difference in the shape of the end of the abdomen, and females tend to have a slightly darker orange color than males.

Photo credit: Ted Wilcox