depending on the size of a average chicken egg bought from your local supermarket or farm they approximately take between 12 - 13 minutes of cooking time in boiling water to be fully cooked hard boiled, start from eggs placed in a pan with cold water and then heated up.
all egg timers out the window, all you have to do is take the egg out of the water with a slotted spoon and place gently on a flat even work surface with lots of space, then with your thumbs and index fingers spin the egg on the spot as fast as you can while taking extra care so that it doesn't spin off your work surface.
if the hard boiled egg is cooked it will spin upright on its end without wabbling. if the egg spins and wabbles without standing upright while spinning then it is not cooked properly.
also note that if upon checking the yolk is cooked but is also has a greenish discolouration around the outside this means that the egg is overcooked.
i hope i have been informative