I agree. Unless you're a expert candler it can be very difficult to figure out exactly what's going on late in the incubation. It should be very dark. Sometimes you can't even see the veins, as they're covered up with chick flesh. Unless they stink, or the hen rolls them out of the nest repeatedly, there's still a chance that they might be fine.
Sometimes powdering the broody butt with corn starch can help prevent the poop from sticking, or can help remove what's stuck on the feathers.
Would putting corn starch on a newborn chick's behind act as a preventive to pasty butt before it happens?