Hello! Sexing a Pacman frog is only possible when they reach sexual maturity. But some of the ways you can tell:

1. Size. Females are larger than males.
2. Mating calls. When in the right conditions, males will croak at night while females stay silent.
3. Males have Nuptial pads. You can see a picture in this thread for what those look like!

There are possibly other minor ways to tell...but I'm pretty sure all of them come around when they become "of age". I know yours is only a baby, but at least you'll be able to tell when it gets older, heh.