Easy to sex when the cocks are in their nuptual plumage. Cocks are bright red all over with a black belly, vent and under-tail coverts (cock left)

The cocks bill is also more red whereas the hen's has black on the top culmen.

Cocks also have brighter rumps, and more bold spotting than hens (cock right)

When cocks are in eclipse plumage sexing is a bit more difficult. They retain the blackish lores accross the eyes which are lacking in the hens.

When looking at the undertail coverts the cock's are edged in white even when in eclipse plumage (cock right)

And lastly, even on an eclipsed cock the spotting on the wings and rump are still prominent.