When the peeper is thinking about hatching, there'll be a certain amount of amniotic fluid that you'll still be able to see when you candle. It'll look like you have a dark mass almost filling the bottom two-thirds of the egg, and a halo of orange light around it. That's nearing the end of incubation.
The chick will have the membrane partially stretched up and around its beak, until it breaks through the membrane. Then it will begin tapping at the surface of the eggshell. That'll look like the picture above, and you'll be able to hear him rapping it. He'll take an age and a half, if he's waterfowl, but he'll finally poke through the shell.
At that point, you want your humidity to already be up. The purpose of the high humidity is to keep that exposed membrane stretchy and flexible. If it's too high too early, too much water will be absorbed through the eggshell and he won't be able to drink it all. He'll get a soft, squashy belly and die. So avoid bringing your humidity up until you're sure he's pipping internally.
After another age and a half, he'll zip--breaking a circle of shell, and he'll hatch soon afterwards.