With some some finches, it is easy to distinguish between male and female based simply on appearance. With others, it's necessary to look more closely and consider behavior. One thing is for certain — if a finch lays an egg, it is not male.
Finches are generally referred to as either sexually dimorphic or sexually monomorphic — in other words, they are either easy to distinguish, or highly similar in appearance.
With sexually dimorphic finches, the males, called cocks, have a more vibrant color, with more elaborate marking, whereas the females, called hens, are not as well marked. Two examples are the most zebra finch and the Lady Gouldian finch.
However, sometimes finches that are sexually dimorphic may have a mutation that changes their coloration. For example, the standard zebra finch is easy to distinguish visually. However, the white zebra finch does not have distinguishable markings.
Another complication is seasonal changes. During non-breeding season, the strawberry finch and the orange weave cocks are indistinguishable from the hens. During breeding season, their colors are more vibrant.
Lastly, some finches are sexually monomorphic. They cannot be distinguished by visual appearance. Breeders must pay close attention to differences in singing, courtship behavior and nesting behavior. With most species of finches, only the cocks will sing. Hens may chirp, but the song of the cock is more extensive. If you watch for courtship behavior, you can spot those finches that are singing and hopping. These are generally cocks, but the object of their attention may be a hen or another cock.
The clearest way to distinguish cocks from hens is that hens lay eggs.