This time of year it is easier to see the difference between the male American Goldfinch and the female American Goldfinch. I posted a photo of a male this past Monday (Male American Goldfinch Molting Into Breeding Plumage). In the winter I would have a difficult time seeing the difference between a non-breeding male and a female. I read that the best way to tell the difference is to look at the underside of the tail feathers. Males have blackish tail feathers with well-defined white spots and the females have grayish feathers blending into dull white spots. (Sibley Guides)
Female American Goldfinch Facts
The American goldfinch is gregarious during the non-breeding season, when it is often found in large flocks, usually with other finches. During the breeding season, it lives in loose colonies. While the nest is being constructed, the male will act aggressively toward other males who intrude into his territory, driving them away, and the female reacts in the same way toward other females. This aggressiveness subsides once the eggs have been laid.
The American goldfinch begins its breeding season later in the year than any other finch and later than any other native North American bird, besides occasionally the sedge wren. This may be related to the abundance of seeds in the late summer months, as seeds represent the majority of their diet.
The nest is built in late summer by the female in the branches of a deciduous shrub or tree at a height of up to 33 feet. The nest-building lasts approximately six days, during which time the female works in 10–40 minute increments. The male frequently flies with the female as she collects nesting materials, and though he may carry some materials back to the nest, he leaves its construction to the female. (Wikipedia)