I have a small clip on book light with a bendable neck that I use to see whether eggs are fertile or not. Knowing whether a hen is sitting on clear eggs can be
These photos are of Collin and Astrid's nest. The pink color of the egg near the light is due to the blood vessels of the chick that has started growing inside.
Since the eggs are laid two days apart, the chicks are at different stages of development, and will also usually hatch at two day intervals. In the photo below, you can see the pinker egg that is more developed, and just below it an egg that is partly yellow with a pink area. To the right of the light is an egg that shows clear. All you can see inside that one is the yolk. It is probably one of the most recently laid eggs, and therefore hasn't had enough time to develop blood vessels yet.
So far none of the Bourke's eggs are showing fertile, but I have noticed that it takes longer to see anything with them. It is important, though, to know about the Bourke's eggs, because it is not good to let the hen waste her time sitting on clear eggs. A Bourke's hen in the box is more dependent on the male to feed her, because she will not come out to eat on her own, like a budgie hen will. So if one of my Bourke's hens lays infertile eggs, I want to get her out of the box and throw the eggs away so she won't be depleted of nutrition and run herself down. Then she can have a second chance to mate with the male and hopefully produce good eggs the next time around. (Eggs don't get fertilized when hens are hiding in boxes!)
The finches are much more independent, and I rarely candle their eggs.