I think also if you google "egg candling" you will find a lot of pictures you can reference. Once you get the hang of it candling is very easy and fast. I can tell within seconds if an egg is ok or not while incubating. I don't think I would open any live eggs if it were me. Seems like a waste. Candling is much less harmful. I personally don't use cardboard I just have a bright led flashlight (think mini maglight size) and put it to the big end of the egg. I can see fine into my eggs.
Good luck with the rest of your hatch!!!!
Good luck with the rest of your hatch!!!!