I tend to candle my eggs on day 5 (sometimes on day 4 if i am really impatient!) and day 10. This is really just shining an LED torch through the shell to see shadows of what is inside. You can get specifically designed egg candlers, but these can be expensive, ad i have never had an issue using a torch. Make sure its an LED though as the older bulb types can get hot and cause issues for your eggs.
I do this to check whether or not the eggs are developing properly. There are many pictures and articles about egg development out there online, its really easy to do a google image search and find pictures of candled eggs at various stages of development. I candle on day 5 to check that development has started, and at 10 to check that they are all still with us. At day 10, if i see no development, i will remove the non fertile eggs from the incubator, as there is a risk that they may explode, contaminating the other eggs with bacteria. I attempted to get a picture for you, but my camera didnt show the veining at all :(
Here is a picture from www.riverbendfarmtasmania.com showing how a candled egg looks at different stages of incubation.
Thanks for visiting!