Distinguishing Between Male and Female Zebra Finches?

I have had two finches for about 3 months. When I first got them I noticed they were the only ones snuggled up to each other. Just today I noticed that one had laid an egg, but it fell to the bottom of the cage. I have no way of telling which finch is the female or male they are both white zebra finches.

My sister did see one of them dance and flop their feathers at the other trying to get on top, the one that laid the egg was the one trying to get on top. It was also cuddling with the other one. The one that was on top has a beak that is darker, but it gives off a lot of female ways.

The other one that is the partner (I think) regected the first one and is also helping to prepare a nest. So my question is, if it's a female or a male? When a couple is ready to nest do both the female and male finch sit in the nest or do only the females? Please help me if you understand my situation. I would appreciate it very so much.
Thank you.

Please help me to tell the difference between the male and the female bird.

I purchased what I thought was a pair of zebra finches last month, one is a standard looking zebra finch the other is a white zebra finch. After a bit of research I thought I had ended up with 2 females, my standard zebra finch has no chest markings or orange cheeks, the white zebra finches beaks and feet are definitely orange not red. They seem bonded, they snuggle and clean each other and sleep together in the nesting boxes.

This week my husband has noticed the standard zebra finch mounting the white one and they have begun filling the nesting box with scrap paper they pull from the cage tray.