you use a small pen light and put it at one side of the egg so the light would be pointed towards your face and you can seen the veins and later on the snake in the fertile egg.
I use a energiser led keyring light.
In the pic is a beardie egg about 48 hours after being layed, i just hold the light to the side but not touching the egg, facing towards the vermiculite.
When they embryo start to develop and get bigger, you have to move the light around a bit to find a spot that will light up the egg (they take up a fair bit of room towards the end)