You can tell that you have overcooked a hard boiled egg by the yolk. It will be harder than usual and take on a blue green color on the outside. It is still fine to eat and the taste should not be affected by overcooking.
I''m not good at cooking. I think you can tell if the egg is overcooked by the eggshell. If there's a crack on the eggshell, then you should stop cooking it?Usually 8 minutes is enough for boiling egg, so watch the time.