Tortoises lay eggs that may or may not be fertile depending on the health of the tortoise and whether they've been bred or not. When laid, these eggs are similar in size to chicken eggs and will continue to grow as the baby tortoise inside grows. Determining whether a tortoise egg is fertile or infertile is easy, but may need a few different attempts as the egg grows.
Pick up the tortoise egg gently and close the blinds or turn off all of the lights in the room to make it dark. It is important to keep the egg in the same position as which you found it, with the same side of the egg facing up at all times. If the egg is turned over the embryo can drown, however it can take a few days for the embryo to establish itself and the sooner the egg is handled after being laid the less of a concern this is.
Turn on the flashlight and aim the light directly at the egg from the top. The flashlight can be placed right against the shell. This will illuminate the egg on the inside, and allow you to see into it. A fertile egg will appear red, pink or veiny inside, whereas an infertile egg will be white or yellow and clear. This process can be done throughout the egg's incubation, and soon a small embryo will be able to be seen. The embryo may even be seen moving, and eventually the light will not be able to penetrate the egg at all because the baby tortoise will completely fill the egg.
Wait a few days and check again if the egg is clear. If the embryo has not attached to the wall of the egg, these veins and pinkish colouration may not appear. It can take up to five days or so for the veins to appear, so try looking for them again in a couple of days. While waiting, these eggs can be placed into an incubation medium.
Observe the eggs every three to five days while they are being incubated. Infertile eggs or eggs that were fertile but have since died may go mouldy, turn yellow or brown and appear dented. The mouldy eggs should be discarded immediately to stop the mould from spreading to healthy eggs, but if the egg is only dented it may be about to hatch.