You have found an egg in the wild or see an egg near an incubator on your farm. It is important to find out if it is alive with a flashlight or through candling because you can then take appropriate action to deal with it. If an egg in the wild rolled away from its nest or looks abandoned, be careful as it may be legally protected. For example, it is illegal to collect bald eagle eggs.
TL;DR (Too Long; Didn't Read)
Poultry breeders test egg fertility by holding it up to a candle and viewing its shadowy insides against the light. This method, candling, can also tell the breeder about the freshness of the egg.
Egg Found in the Wild
Observe the egg you found for a few days. Assess if there is any activity such as birds traveling back and forth to the nest. This could be a sign the egg is being nurtured and therefore still alive.
Touch the egg gently with the back of your hand when you find it. If an egg is alive, it will feel warm. If it has just fallen from a nest, it may also be warm, yet still be dead. Therefore, you will need to perform additional checks to make sure it is alive.
Inspect the shell of the egg. It should not have any cracks or flaws in it if the egg is still alive. Look for thin, hairline fractures or other distinguishing marks that could show damage and indicate it is no longer alive.
Egg Found Near an Incubator
Consult a vet who has specialist bird knowledge. A wildlife expert at a bird sanctuary or zoo might be able to help you clarify if an egg is still alive.
Consult your local wildlife department or contact the U.S. Wildlife and Fish Service before disturbing the egg to find out if you can pick it up. For example, in Ohio, you can apply for a Scientific Collecting Permit to collect eggs for research.
Remember it is illegal to candle robin's eggs.
Check the eggshell for damage. Large cracks or small areas of webbed cracking could indicate serious damage inside. It should have a smooth, unmarked shell if it is still alive.
Shine a bright flashlight through the egg in a dark room, and look closely at the inside. If the egg is alive you will see veins running through it. The process of removing dead or rotten eggs during incubation that uses this method is candling.
Compare the color of the yolk with a regular infertile egg by candling each egg separately. If the egg is still alive, the yolk will be pale and not as orange as the infertile egg.
Watch the egg as it incubates. Identify signs of life such as slight wobbles when the animal inside is moving around.