Have you read this? (or parts of it, might be way too much info lol) The beginning might be useful.
You could try float testing them if you check them over and you're SURE they have no cracks AT ALL. Here's a few links on float testing.
I usually hold the large end of an egg (that is close to hatching) up to my ear, and if I hear any cheeping or other sounds it is alive but if I don't hear anything it could be a dud, or it could be taking a nap. Personally I don't get rid of eggs until at least day 25, because if the incubator temp is slightly cool then they can take longer to develop.
Or you could try this.