Unless the first egg was in a very hot spot in the incubator, they should have all developed at relatively the same rate. So usually after the first few chicks hatch, I start to worry a bit about the ones that havent pipped yet (because they may be over 24 hours behind.)
If you have a way to candle the egg, shine the light through the fat end of the egg, where you should see the air cell. If you see a beak sticking up into the air cell area and movement up in there then it has internally pipped. Also, you can hold the egg up to your ear and if you hear peeping then it has internally pipped