Cock has got a much wider crown, and in pure birds the mandible appears more lavender from the top than that of the hen. These show pink instead indicating that they are not poor bloods.

 
Side profile not such a distinct difference as in its cousin the Java Sparrow, but the beak appears slightly heavier in the Cock (on the left)

 
The cocks tail (on the left) is also narrower and more pointed in addition to being slightly longer than the hens. Also shows up sooty-black in good sunlight compared to the hens charcoal colour.