Something I like to do when I'm at an event is share little tidbits of information about certain insects that might be fun or interesting or sometimes just plain odd. So I'm always scouting the internet to learn new stuff. When I first started insect pinning, one of the first things I learned is the easiest way to tell the difference between the male & female Danaus plexippus, or Monarch Butterfly. And the funny thing is, once you know, you'll always look for this one very obvious identifying feature.
As you can see the male has a little black looking dot or patch on each of it's hindwings. These are called the androconal patch. At the base of these androconia are tiny sacs that contain pheromones used to attract females for mating. They have tiny hairs on the edges of their scales that disperses the pheromones.
Another, fairly easy way to tell them apart is that the wing veins are thinner on the males of this species. Though, you'd most likely need to have a male & female together to really tell the difference.
And that’s it! All you have to do is spot those spots & you’ll know whether you have a male or female monarch butterfly.