Finch Breeding - Finding a Pair

Zebra Finches are easy to sex. Males can be visually distinguished from females. Male Zebra Finches have a black breast bar, chestnut flanks speckled with white, orange cheek patches, a scale-like pattern under the chin, and of course the red beak. Females on the other hand are totally lacking in any of these male markings and their beaks are a lighter and more orange color.

There are Zebra Finch color mutations that have less-defined sex markings, so if you really can not see any distinguishing markings on your bird, look at the beak color. In all but one Zebra Finch color mutation, the beak colors will still be red or orange, depending on sex.
source: Myra Markley