A fairly easy species to sex visually. Very similar to the Common Waxbill but they have a red rump and red in the wings.

The cock's head is slightly bigger and the buffish-white cheeks are more noticeable when viewed from the top (cock left)

In side profile the cock also shows more a more distinct white throat and ear-coverts (cock left)

But by far the easiest way to identify the sex is by comparing the red under the tail which is more prominent in the cock (cock left)