You will see mostly dark under the air sac. If there is room in the egg you may see movement, sometimes not much---don't panic. Sometimes on day 18 at LD, I will have a bump in the air sac where the chicks head is protruding. Sometimes it is completely through the sac and moving around. It's kind of neat! I usually have 1 or 2 of 100 through the sac when I lock down. Seems I always have several broodies and I'm guessing they are getting a jump start on a few of the eggs I set, thus a couple of early chicks jumping around the hatcher. Have fun!!!
This is a good time to pull the "bad" eggs too. I always candle at LD