Typical mannikin style, there is a difference in the beak area. Viewed from the top, the cocks bill is much more red, especially at the base where it touches the forehead. The red eye-ring is also more obvious when viewed from the top. Hen is seen dead on and the rings don't show, the cocks head is tilted slightly but the right eye-ring is still visible (cock on left)

In side profile the cock (on left) also has much more girth to the beak, and again noticed the brighter red colour. The eye-ring is also more pronounced and slightly brighter than the hens.

When beaks are viewed from below the cocks are still brighter (cock on left) For interrest sakes, I have only ever seen 3 Java's that don't have the white patch under the chin. Pure wild birds do not carry this trade and its probably due the presence of the pied gene.

 Cocks also have more sooty-black tail-feathers (cock on left)

Works on all colour mutations. Perhaps more destinct on the white mutation. In fawn birds the brown on the head is much darker in cocks as well.