To check if your chicken eggs are fertile or not, you can put them through a procedure called Candling. Candling is the process of shining a light through the egg to see if there is any sign of an embryo inside.
The fertility of eggs cannot be determined before you incubate them, so after 2 to 3 days in the incubator, you can candle white-shelled eggs to see if embryos have developed.
Eggs that are damaged do not hatch and often develop odors and they should be removed when found.
White-shelled eggs may be candled by placing a light bulb under a box. Make a hole slightly smaller in diameter than the egg through which light will pass. Place the egg over the hole, if a cloudy spot is observed, this can be assumed to be a growing embryo. If the contents of the egg allows light to pass uniformly through it you can assume that the egg is infertile. If an egg is candled at 7 days or older and is absolutely clear it is dead or was never fertile.
If you prefer you can buy a Candler, which is a sophisticated product specifically designed to show you if the eggs are fertile or not.
Here is an example of a Candler below:
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