Gently take an egg into the bathroom, turn out the light, and shine a flashlight through it. The flashlight should be touching the egg. The egg should be sort of between the flashlight and you. It takes a bit of practice to get it right, but once you do you should be able to see if there is anything in the egg or not.
Unless the eggs are brown or green. I never HAVE been able to candle any opf the green eggs, and brown is harder to see into than white!
Lastly, always handle an incubating egg gently. Don't shake.