Many answers come from quietly observing the nest box from a hidden position in the aviary.
The remainder of the answers come from at least one daily look in the nest box. Recently I visited a man's aviary to sex some birds for him. On seeing a hen come out of her nest, I asked him if she had babies. His reply was that he didn't know because he was afraid the birds would quit the nest if he looked in the box. Curious to see, I took the lid off and looked in the box.