Candling mallard duck eggs?

A duck layed her eggs in my yard and recently something got into the nest and the mother stopped caring for the 3 eggs tht were left.. I don't know how old they are and when I candle them I don't know what to look for. I don't even knoe if their alive or fertile how do I check. I also realized by candling tht one of the eggs yolk is getting larger is this bad?