Copyrighted material used with permission. I read that the female has a neck of round dots. My bird has a necklace, not of dots. The male's sweet, canary like song is one way to tell the sexes apart, but it is easier to rely on appearance of the adult birds.
Males have grayish-green plumage with yellow stripes and bright yellow breasts.
Similarly colored, females are duller overall and wear a necklace of dark spot around the neck. The only difficulty occurs when sexing juveniles, which resemble adult females. Green singers also appreciate greens, a favorite item with serins and ripening seedling heads, such a chickweed or plantain. To please breeding birds, augment this menu with a few small mealworm and soaked seeds. Offer varied nesting material, including grass, moss, burlap, rootlets and some fine materials, such as soft feathers or plant down, for nest lining.