Instantaneous heat and instantaneous humidity are pretty irrelevant unless you are so hot you cook the eggs. Plenty of us candle that number of eggs with no problems. It’s average heat and humidity over the entire incubation that’s important, not a snapshot in time.
Anyway, no there is not a good way for us to tell if they are fertile. There is a way a laboratory can take a sample of the egg white and tell if certain hormones are present to determine if it is fertile, but that’s not practical for us.
If you can tell which hen is laying which egg, you can open some and look for the bull’s eye. If one egg she lays is fertile, the others probably are also. This doesn’t come with a guarantee though.
Just because an egg does not develop does not mean it was not fertile to start with. Health and nutrition if the parent flock has an influence. How and how long you store the eggs can make a difference too. With a fertile egg, the embryo is alive when the egg is laid. If the egg is stored too hot, too cold, for too long, or cycles between hot and cold, the embryo may die. That has nothing to do with whether it was fertile to start with or not. You might want to look at your storage methods if it appears to be all the hens’ eggs having this problem.
If you can determine which hen’s egg are not developing, you may get some clues that way. There may be something specific to one of the hens. For example, some chickens with extremely thick feathers sometimes have problems with fertility. The thick feathers keep the rooster from hitting the target. This is a real possibility with show quality Orpington, Cochin, and other thick feathered breeds because of the extremely thick feathers these chickens can have. Some breeders trim the feathers around the vents on their breeding birds to increase fertility.
Over the internet it’s impossible to be sure what your problem really is. In person it’s not always easy. In general, if an egg does not develop or dies during the first week of incubation, the cause is something that happened before the eggs were incubated. If it dies in the last week, it probably had something to do with the actual incubation. This isn’t always true but it’s a good clue of where to start looking. Good luck figuring it out.
