The embryo develops at a predictable rate inside the egg. By shining a bright light through the shell you can see how it is progressing. How easy it is depends on the thickness of the shell, Mandarin ducks are easy to see, most Geese are very difficult. In general, darker coloured eggs are the hardest to examine.
Some mobile phones have powerful torches, and with a loo roll you have a ready-made candler. There are several dedicated egg candlers available to buy.