Walter Burgess: Sexing 101 - species # 11 - Lonchura griseicapilla - Grey-headed Silverbill (Pearl-headed Silverbill) :

The most difficult Silverbill to sex visually, surgical or DNA sexing is advised, but there are slight differences when compared to each other. Unlike most of the Lonchura there is not much difference when viewed from above apart from the cock (left) being slightly darker.

When looking at the face there is also a difference. The hen's spotting is more random and patternless (exept for right under the eye) whereas the cocks is more structured overall, with most dots forming a line of sorts. Also notice her breast colour is more fawn and his is more chocolat- brown (cock right)

Lastly, when looking at the chest area overall the cock shows a more chocolate-brown colour in comparrison with the hens fawn colour. The difference is more evident if you view the birds from a distance due to the refraction of light (perhaps hold the phone further away!)