By day 15 the eggs should be completely dark at the botttom (the pointed end) and you should be able to see the aircell at the top. Always candle with the flashlight on the top of the egg. You may be able to see a tiny bit of movement where the aircell and darkness (this is the chick filling up the egg) meet. Be very careful handling the eggs. If you have a pair of latex disposable gloves this will help you grip the eggs.
Day 18 is the day you quit turning and raise the humidity...commonly referred to as lockdown.