A very hard species to sex visually. Not so much difference when viewed from the top as in in other Lonchura, but in side profile the the Cocks bill is shorter, stubbier and thicker at the base. The hen's is more pointy.

 
The cock (on the left) has a slighty bigger area of yellow-brown around the nape.

 
When viewing the markings on the underside, in the cock the markings almost form destinct rows, whereas in the hen its more random and patternless, especially on the flanks (cock on left)

 
When viewed in good sunlight the cock's tail (on the left) will appear more sooty black than the hens.