To confirm if an eggs are fertilized or not, this light will let you see. You just can't tell before then. You can mark your eggs with dates in pencil so that you can keep track. This light was very helpful and was really fun to watch the growth of the eggs. You can check about once or twice a week to make sure that they were still growing. Features -Color:Multi -Material:Aluminum Alloy, ABS -Size:12.00X3.60X3.60cm/4.72X1.41X1.41in -Note: not included battery -Professional and practical design with beautiful appearance, easy to carry and use. -This portable handheld led egg inspection tool is suitable for incubating eggs of different types, including chicken, duck eggs. -After the incubator has running for a few days, it's possible to check the eggs with a cold lighting to see inside the shells. -When the lamp is , the hatching degree in the egg can be clearly seen, and the hatching degree of the egg can be easily judged. -Light egg candler is a product that checks whether the egg is fertilized and grows healthy. Package list: 2 Egg Detector