Visual inspection is all that is really needed to tell if a python egg is fertile. If it is plump and white then it is most likely fertile. If it is small and yellow then it is probably a slug. For those breeders that are still unsure, or for those weird looking eggs, candling is an easy technique used to determine if an egg is fertile.
Candling eggs is really pretty simple. Use a small flashlight (a Mini Maglite works well) and place it at one end of the egg. When the lights are turned off in the room, the egg should appear to be illuminated.
Fertile eggs will have a nice pink glow. Blood vessels should be apparent.
Slugs can be identified without the candling. They are smaller, tougher, and have a yellow to tan color. Occasionally a normal looking egg (a white one) is laid but is not fertile. I have had female ball pythons push infertile eggs out of their clutches only to coil around the fertile eggs. Sometimes weird looking eggs are laid. This is where candling helps out. When candled, a slug (white or yellow) will be illuminated with a yellow glow instead of a pink glow. Blood vessels will not be present at all or will be very thin and weak looking.
Candling is not a necessary process when hatching pythons. Rather it is a technique that gives the breeder a piece of mind. On a side note, I have used candling to pick out albinos from hets while they are still in the egg. The eggs with albinos appear normal for most of the incubation period. When candled during the last few weeks of incubation the eggs with albinos appear to be much brighter than their normal appearing clutch mates. I am sure candling can be used to spot many other morphs before they hatch!