A simple way to tell if a duck egg contains a live embryo is through the process of candling. Candling an egg simply means shining a bright light through the egg and looking at the contents inside the shell to verify a living embryo.
One of the most important signs to look for when candling a duck egg are veins. Eggs with living embryos usually feature clear veins that are visible throughout development. Another sign to look for when candling a duck egg is movement. Duck embryos begin moving around day 12 of development and are often stimulated by the bright light used during candling.