If you are not accustomed to candling, at days 7-10 only candle for "fertility". In other words, only pull the clear eggs.
But I understand where Clint is going with his advice. As the older eggs will be less hydrated, thus have larger air sac.
There is a lot of information on "egg development, candling chicken eggs, etc" on the net. Many sites have actual pictures that will give you an idea of what to look for. The key, IMO, is to use a strong led light. I have a little flash light that cranks out some 160 lumens that works very well.
Best of luck.