The easiest of the 3 silverbills to sex.

 
When fiewed from above, the cock has a noticably wider bill and the hen's appears more pointy (cock on left)

 
In side profile the cock upper mandible is more bulky than the hen's. His lower mandible also appears longer than the hen's and has a different shape to hers (cock left)

 
The cocks are much whiter in the front, but the best area to compare is around the face. He is much whiter, noticably around the lores, where she is more brown. More obvious when the birds are in motion than in the hand.

 
Lastly, I have noticed that cock's have a wine-red sheen to the tail, where hen's have a more mustart sheen (cock top)

 
Strangly I have never seen this noted in any literature, but its quite obvious, more so in this pic.

 
And then just to top it off, some cocks (not all) will carry a bit of pink in the rump (again no mention of this in the literature) I have always thought that the Cherry Finch must be related to Silverbills, and since I only saw live Indian Silverbills some 3-4 years ago, this pink colour has convinced me that it must be so.