it's really a tough call, and more of a gut instinct. But generally, they are still absorbing yolk, even after external pip, so helping too soon can be disastrous. Absorption happens much quicker and more efficiently while he is inside that shell.
having said that, I hope it doesn't come to this, but the membrane should stay papery white to wet-looking gray. if it starts looking brown, he may have stopped moving (they constantly make little movements and stay lubricated) and may be drying out. That would be the signal to possibly start assisting. Slowwwwwly, start by chipping away around the pip hole and find his bill. Free breathing is first priority when assisting. Then moisten any exposed membrane with vaseline or coconut oil, and look for active blood vessels. You do not want to rip thru any blood vessels. Any bleeding at all, stop! and wait until vessels recede. keep in mind that usually once you start assisting, it's a process too, and must be done slowly. But once you start, you will likely have to continue (over a period of a few hours) until it fully hatches.