Although it is not necessary to test eggs for fertility, you can eliminate the eggs which are not going to hatch by doing so. It is also an interesting phase of the project, since it is possible to see clearly the developing embryo.
Get a shoe box or a box of similar dimension made from wood or tin and cut a hole in one end about one inch in diameter. Use an extension cord and mount a 60-watt bulb in the box. Darken the room and hold the large end of the egg to the light. What you will see depends mostly on the age of the embryo. It is difficult to see much development until the 4th or 5th day of incubation.
The first parts of the embryo which you will be able to see by candling will be the head and eye, and they will appear as a dark object. If the embryo is alive and circulation is established, the contents of the egg will have a pinkish color or cast. But if the embryo is dead the contents will appear muddy or brownish.
The live and growing embryo will eventually occupy all of the interior of the egg and will not transmit light; thus, it will be impossible to see anything but the air cell at the end of the incubation period. Infertile eggs and early dead embryos can be detected readily because they appear clear.
Removing the eggs from the incubator for candling does little harm if you handle them gently. It may slow up development of the chick, though, depending upon how much the egg is cooled. Generally, if the eggs are removed from the incubator two or three times for a period of no more than 15 minutes each, such cooling will make little difference in the total incubation time required for hatching. On the other hand, if the eggs are cooled for several hours because of power failure or some other reason, hatching time may be delayed. It is as important not to cool the eggs too long as it is to avoid overheating.