That's why I like the hens to hatch for me. I would constantly be watching a 'bator.
When the hens get off the nest I look over the eggs at that time and sometimes we see one that is making it's way out but for the most part I just watch the hen. I like that so much better.
How are you hatching these eggs? Are they laying on their sides, or in a carton? If they are on their sides, have you seen any rocking? Do you hear any peeping?
I don't think it would harm them to candle, provided there aren't any pips. They won't dry out the membrane as long as the shell isn't broke. You need to look for any that have the air sac pipped, and see if any are moving.
Depending on the temp and humidity during incubation, they could take up to 23/24 days to hatch. After that I think it would be a lost cause.
If they have pipped the air sac, the air sac won't be as uniform as it was. You will be able to see the chicks head/beak in it. You could also hold the egg up to your ear and tap the side with your fingernail. If it has pipped the air sac, it will probably peep at you. If it is not asleep, you should be able to hear it tap back.