When leaf insects are adults they’re very easy indeed to sex. The males are small and thin with long antennae and wings that enable them to fly well. The females, in contrast, are far larger and more rounded with only small wings and are unable to fly any distance.
However sexing younger insects can be rather more of a challenge. When leaf insects are tiny it is near impossible to tell the males from the female but as they grow up things become a little easier.
In essence, the juveniles start to take on the general body shape of an adult insect with the males becoming longer and thinner while the females become rounder and fatter in appearance.
The following photos illustrate this with a female at the top and a male at the bottom. Note the difference in abdomen shape.