You need to candle to see if the air cell is delevoping correctly. In needs to be a fairly large size by day 18. You can see the air cel with most flashlights. I use a common LED and can see thru my brown eggs but no the marans eggs.
You can also see the embryo developing and know it's growing.
Diagrams of air cells, duck and chicken:
http://www.poultryconnection.com/quackers/aircell.html
My first time hatching, I loved checking the eggs, a few every couple of days. I washed my hands very well everytime and dried my hands with a new clean towel. I also learned that extra handlind can result in broken eggs. THey seem to become more fragile and porcelin-like as the days of incubation progress.
GL