A very difficult species to sex visually. In fact it's the most difficult in the Estrilda genus. Differences are very subtle so DNA sexing is advised.

The cock's head is slightly bigger (cock left)

And he is also a little darker on the crown (cock right)

But the most accurate way is to stick to the family traits and look for a difference in the ventral region. The cock is a little bit darker than the hen, notably the undertail coverts and most noticeably in the area between the back of the legs and the base of the tail.