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:Silver -Material:Aluminum Alloy, ABS -Size: 12.00X3.60X3.60cm;/4.72X1.41X1.41in; -Note:Battery is not included. (3 x AAA Batteries) -Our egg candler provides cool and strong light, no heat damage to the embryo and is perfect for monitoring egg development. -In a dark place, put the egg in the mouth of the lamp, and judge whether the egg can hatch into a chick by observing the inner outline of the egg. -Brighter and cooler light sources are available which do not carry the risk of harming the development of the embryos. -When checking whether the egg to be incubated or not, just put the bigger side of the egg to the upside, turn on the led egg candler and aim at the tops of the bigger side. -Lightweight, portable, and easy to carry and use, it can bring you great convenience. Package list: 1 Egg Detector