When trying to hatch duck eggs it is important to know if the egg is fertile or not. While knowing how many eggs may hatch will help you plan for the size of your brace of ducks, it will also prevent fouling the incubator as infertile eggs will rot and stink. It can also be enlightening because you don't often get to see an unborn living creature.
Set up your torch so that you don't have to hold it and can comfortably look at it. You don't want the torch to roll when you're candling the eggs. It should be bright enough to illuminate through the shell but not add a lot of heat.
Remove your duck eggs from the incubator and place them in an empty egg carton near the light to prevent them from rolling off the worktop or table.
Draw the blinds or otherwise exclude light from outside. Turn on the torch and turn off the room lights.
Hold an egg up to the torch, look through the egg and look for shadows or the red-coloured blood sac.
Look for clear eggs. Dispose of them since they are not fertile.
Look for eggs with dark bodies or reddish spreading nets of blood vessels. These eggs are fertile, return them to the incubator.
Determine whether there is a "blood ring" where the blood has pulled away from the dark embryo. Dispose of these failed embryos. If the blood vessels are in a net and in contact with the darkened shadow (the embroyo), it is continuing to feed oxygen to the embryo. These are still viable, and you can return them to the incubator.
If the torch is wider than the egg, you can mask part of the light so it's not blinding, using masking tape, cut-out cardboard or other similar materials. If you can't see anything, but the egg is less than a week old, return it to the incubator and check again in a few days.
Handle the eggs gently and don't hold them to the light for long. Don't look directly into the light. Never eat an egg that has been incubated. Discard the egg as it could contain salmonella.