A very easy species to sex visually and my favourite Firefinch. The cocks are mostly red. Be wary of buying young birds, as birds presumed to be hens will start showing red spots proving that they are young cocks (cock left)                       

The cock also has a slightly brighter yellow ring around the eyes. Notices that hens have red lores (cock left)                       

In the southern form kept in South Africa the red is replaced with brown on the crown. In the northern form the red extends well over the head (cock left)                     

The sexes are virtually identical on the back and wings in local birds, but in the northern from the cock will have the red extent onto the mantle, back and wings (cock left)