Females can very often have darker throats. More pigment to them at very least. Esp when it comes to Ceratophrys Cranwelli. A good way to tell is by the presence of nuptial pads on the males front feet. Females might have small bumps here, but nothing obvious like the male. These dont usually fully develop until almost adulthood either. I have had males and females w almost black throats and both that had no darkness to the throat. You can try misting your frog w water a few times a day to try to get him to "croak" or ''call". Or even play sounds of other frogs calling. That usually stimulates males to call.