You can tell when they’re an inch long. It looks very female to me. Short pelvic fins, purple belly, and the bulge is too far back to be food, suggesting the female rotundness that males don’t get.
A picture with the pelvic fins out would confirm.
A trick with many colour forms of P pulcher is o look at the very back of the dorsal. Females often have a dark spot there - males don't. You can often sex them very young with that, although it's clearer in their relative, the real P kribensis.