It's a pretty easy call with white eggs or light brown eggs.  However, with tinted or dark brown eggs, it isn't easy to see whether or not they're developing.  I'm not an expert, but unless I detect a bad odor, I plan to keep all mine in the incubator.  Last time there was one egg that I was sure wasn't doing anything because all the others had hatched and it was still there - but I couldn't see inside because it was a dark welsummer egg.  I kept the bator going anyway, though, and decided to wait a couple extra days.  Two days after all the others hatched, out popped a chick.  So I don't give up on an egg unless I'm absolutely certain it's a dud.