How do you now if a ducks eggs have a duck inside?

If you take a light into a dark room and shine a light directly into the egg, you can take a look inside. If you see veins and a small blob within, most likely the small duckling inside is not dead. If you check up on it regularly and can see the central blob in the middle progressively grow larger, it's definitely alive. However, if you look inside and see a red ring inside of it, that is a sign that it is dead and will not continue to grow.