A hatcheries advice to my "unknown date of hatch" was to listen for peeping. When I hear the ducks, then stop turning them and add the humidity. Do they always make sounds or are there other indicators? Do you know of any good candling websites that show what eggs look like after day 20?
http://lancaster.unl.edu/4h/Embryology/CandlingPhotos.shtml There are some pretty good pics and videos there. Good luck with your hatch! Yes, you can hear them chirping once they've broken into the air cell and begin to breathe the air there. BUT you might NOT hear them either. If you candle an egg where the chick is in the air sac, you'll see the beak of the chick poked up through the air cell, OR the whole air cell pretty much filled in as the chick begins to pip.