You can only guesstimate how far along by candling.
If the egg is "all dark" with a large air cell then you can be sure it is pretty far along. If you can see a moving mass swimming around inside then you have quite a way to go.
If you think they should hatch in the next couple of days listen quietly for peeps from the shell (or, I am not recommending this but I did it to check a dud egg yesterday, set it on a solid surface and watch carefully for movement). The "dud" egg moved and promptly went back under mama...that was yesterday and today the egg has pipped and is making little tapping noises.