Perhaps the strongest phenotypical discriminator of Common Waxbill gender is the colour of their vent and undertail coverts. As can be seen in the image below of my eight specimens, even young cocks have deep black in this area, extending from the coverts to just past the vent area, and sometimes even extending up into the belly area. The brown areas on the belly of the cocks are also a darker shade than that of the hens.
Hens on the other hand tend to be much browner in the area of the vent and undertail coverts. Although you can also find black areas here, especially on the underside of the tail feathers, their vent areas tend to be heavily diluted with brown.
The images below show three adults (bottom right), age unknown because they were imported as wild-type specimens. The remaining five specimens were all only two months of age when these images were taken, so still quite young. However, those marked as cocks were practising their singing and already had deep-black coverts and vents.