Understanding when a chick is ready to hatch is difficult. An egg takes 18 days to develop only after the hen begins incubating the egg. Usually, the hen will start sitting in earnest on her eggs after she has laid a couple of eggs, which are laid every other day. So, even if you record the date the eggs are laid and number the eggs, you will not necessarily be able to calculate when the egg is truly due to hatch.
Begin to watch eggs closely after the 18 day window has passed for signs that an egg is ready to hatch. This is particulary important during the summer months or when the air is dry which can cause the membrane inside the egg to dry out and attach to the chick making it difficult for the chick escape.
Eggs that are ready will be darkened and solid looking. The air space in the egg will be quite small. With a pen light, the egg will reveal less liquid inside. Even after observing these indications however, if you decide you need to assist, you may be too early and the chick will die. Examine the egg closely to determine if there are any chips or cracks in the egg that the chick may have started. Usually, if all is going well, the chick will have little difficulty escaping the shell. Observing an egg with a pip in it probably indicates that the chick needs help.
The best way to tell if a chick is ready to hatch is by holding the egg up to your ear. If you can hear the chick peeping inside, it is ready to experience life. At this time, I always intervene.
To assist an egg hatching, use a penlight to determine where the air space in the egg is. Use a fingernail and gentley chip away the egg shell. At this time, the head of the chick can be seen, gentley peel away the rest of the egg. If the membrane is stuck to the chick, use some warm water to gentley remove it. The umbilical cord will be a bit bloody and attached to the membrane/shell. Carefully pull the umbilical cord free. It will dry up shortly. Return the chick to the nestbox. No need to dry the chick, it will dry on its own. If the chick is exhausted from trying to escape the shell, it may need time to rest. It may be weakened not able to cry much to be fed by the hen. If this is the case, and the mother hen is an inexperienced feeder, you may wish to put your newly assisted hatch baby in a nestbox with an experienced hen if she also has newly hatched chicks.