To confirm if an eggs are fertilized or not, this light will let you see then by showing the veins. 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:Blue -Material:Aluminum Alloy, ABS -Size:10.50X3.20X3.20cm/4.13X1.26X1.26in Note:3 x AAA batteries(not included) Light:cold light -Professional and practical design with beautiful appearance, easy to carry and use. -The Egg Tester uses high-penetration and high-brightness lamp beads. -After incubator has been running for a few days, it's feasible to see the eggs with a bright light to see inside the shells. -When the lamp is lit, 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. Package list: 1 Egg Detector