Fairly easy to sex visually.

Cocks have a bolder head than the hen. In good light it will also appear more "blue" than the hens (cock left)

He's bib is also bigger and more triangular in shape in comparison with the hens more pear-shaped smaller bib (cock left)This is easier to see when the birds are perched and relaxed. There is also 2 mutations available in SA:

The fawn, which just like the Grass finch comes in a few shades. Some are reddish-brown while others are very light almost beige. The bibs are chocolate brown (note the smaller bib on this bird indicating that it is a hen)

And then Cream, an all-white bird with the bib and trouser stripes in cream. Note that other than the mutations of the Grass finch that don't affect the beak colour, as the Parson has a black beak the colour does get affected. In fawns it goes dark brown and in creams it goes pink. This doesn't mean the bird is a hybrid as is commonly thought, it's simply the affect of the  mutant gene. Hybrids will show red or yellow to the base of the bill.