I don't know...

First off, if you bought the egg at a supermarket, those eggs are not fertilized. Now, if you got the egg from a chicken farm and think that it might be fertilized, you candle it. Candling is a way of looking inside the egg to see if an embryo exists.

You need a bright light and a dark room. The poultry industry uses fancy equipment but you can get by with a small penlight or a Minimag flashlight.

Wait about a week after the egg was laid. Get in a dark room, turn on the flashlight, hold the egg at eye level about a foot away, and hold the flashlight very close behind the egg so you look through the egg. You’re looking for tiny red veins connected like a spiderweb and a small dark spot. This is the embryo of a fertile egg. Sometimes, you can see it best if you shine the light on the big end of the egg, where the egg sac is. If you don’t see the web, put the egg carefully back in its nest and check again in a few days.

You can spread bacteria to the chick through the egg by handling it. So, wash your hands first and make sure they’re warm and dry. Never shake the egg.

http://www.wonderquest.com/eggs-fertile.htm