To candle the eggs, you hold them up to a very bright light -such as a flashlight- and this will allow you to see the shadows of the chick and blood vessels inside.
Generally mother ducks will hiss and even bite if they are there when you try to touch their eggs. It's therefore best to check the eggs while the mother is off the nest to eat and drink; when my ducks had nests, I would use this time to candle, check on, and even mark the eggs and remove those that weren't developing. You can also scare them off the nest so that you can work with the eggs as the hen will return to the nest when you leave, in my experience. I would however avoid scaring the hen if possible.
It's also a good idea to keep an eye on the nest at hatch time, since mother ducks tend to leave behind eggs that are still alive but have not yet hatched if they have a few that have already hatched. I've raised several such ducklings, and the longer they are out there without mom the longer they have to be exposed to cold and pests that might do them harm.
