Okay. First of all, it all comes down to how long the egg has been incubating.
I just candled mine today, which have been in my bator for a week now.
All my eggs have a chick developing in them.
Now when an egg is candled at that point in time, what your going to look for is #1, an air sac, these are on the small end of the egg. #2, you can usually see a tiny black spot in the middle of the egg. And #3, some veins. At this point in time... if the egg is empty, nothing is developing.