Why won't my finches share a nest? How to tell male vs female?

When zebra finches are all white it is sometimes hard to tell if they are male or female as they won't have the characteristic cheek markings, beak coloration or tail patterns. The two pictured look like they could possibly both be female, but it is hard to tell due to the distance from the camera and the lack of markings present (males will sometimes still have darker cheek areas, a stripe on the cheek, or a darker red beak).

Males will also sing almost constantly that sounds quite different than regular chirping, and will also "tail waggle" when they are wanting to/attempting to mate with a female. If none of your birds are doing this, it is possible they are all female.

Both males and females will show interest in nests and will go in them to sleep at night even if they aren't raising babies. If one is being left out, you can add in another nest or sleeping area to see if it helps.