You can typically shine a light through the egg at night and see the chickens developement. they will often move when exposed to heat or cold. if the chicken has been exposed to any harsh conditions, vibrations or the egg is discolored or soft the chicken typically will not hatch. incubators can cook them alive!!
You need to let the mother hen sit on them for a few weeks. Usually 2 to 2 1/2 weeks long. IF after that set of time, she'll let you, you need to pick up the egg, holding it to the light, you should see a baby chicken.
You can't really tell. After the chicken sat on the eggs for like 2-3 weeks. Take egg from nest hold it up to a bright light, you should be able to see inside the egg with a baby chick inside. If chicken doesn't sit on certain eggs it is possible it may not hatch.