In general, most farmers and poultry owners will candle their eggs twice. The first candling should take place after about one week of placing the eggs in the incubator, but this may vary a little depending on the type of egg and color of the shell. Darker shelled eggs should be left a little longer than lighter eggs before candling, as it can be harder to see inside them. At this mark, you should be able to see a spot inside the egg with vein-like branches spreading out from it. This is the embryo. The second time you should candle is at around the 2-week mark after placing the eggs in the incubator. At this point, the egg will probably have gotten darker, the spot in the center will have grown larger, signifying that the embryo is almost ready to hatch, and the veins should still be visible too.