You can easily distinguish a Male gouldian finch to a female gouldian. 1st is to compare The birds, Males tend to have brighter color than females. 2nd is to compare their masks, males have solid mask than females. 3rd, also take a look at their pin tails, males have longer pin tail than females.
Female gouldian’s beak will turn black o grayblack if it is in breeding condition, this is only true to normal gouldians(green backs).
Another thing is if u compare the yellow part under the breast of gouldians, males have intensified yellow color and females tend to have dull yellow coloration.