Things You'll Need
Instructions
Remove the egg from the incubator, and start a timer if necessary. Though duck eggs are extremely forgiving of changes in temperature and humidity, you must still take care to return the eggs to the incubator as quickly as possible.
Turn out the lights in a room, and hold the egg between you and a flashlight. You should be able to see the light shining through at least part of the egg.
Look for a black spot and blood vessels in the large end of the egg, which signify a fertile egg. If you hold it still, you can see a heartbeat in a living egg after about three days with the naked eye.
Check on development over time, and watch for blood rings or embryos that aren't growing, which generally signify a dead egg. Remove these eggs and discard them so they don't infect the rest of the clutch. Check the eggs at least on days three, six, 12 and 25. A healthy egg will hatch after about 28 days.