How do farmers find out if they have a chick growing in the egg?

We use a tool called a candler. A candler is like a very strong flashlight that we put against the egg while we are in a dark room. The light shines through the egg shell and we can see changes on the inside like shadows. Dark shadows show us that a chick is forming...

If the egg has no shadows at all, we know that the egg was not viable and we remove it from the incubator.