As the egg yolk thickens it will start to change shape and color. Some spotty discoloration as the yolk heats up is normal, but if the yolk starts turning pale yellow, you've gone too far. There will always be some carryover cooking, so if you're unsure, turn the burner off or remove the egg from the pan and let the egg sit for a minute or two before serving. This is a low, slow cooking method so as long as you don't wander away from the stove, you should be able to pull the egg off the heat before the yolk completely cooks through.