How can you tell if a caterpiller is a boy or girl?

All monarch caterpillars look the same, whereas the adults have different abdomen shapes and the male butterflies have to black spots that the females lack. That's why we can distinguish the adult males from the adult females.

If you were to dissect monarch caterpillars, there would be at least one difference. The males have two bright red organs that the females lack, but there is no way of knowing which are male and which are female just by looking at the outside of the caterpillar.