A fairly easy bird to sex visually provided the pair is from the same sub-species as there is a lot of variation depending on the area the birds came from. It's the only member in the Uraeginthus genus that is sexually monomorphic.

The cock's blue is much richer in colour than the hens. His bill is also more pinkish than the hens (cock left)

The pinker bill is even more obvious from the top as well as the brighter blue colour.

Lastly the cock's blue feathering is more extensive than the hens, notably on the flanks. Usually more obvious when the birds are viewed from a distance probably due to the refraction of light