I can make an egg over easy, but somehow I can't get over medium. I've tried different methods like lower heat, longer time and high heat, shorter time. At best I end up with a yolk that's overcooked on the edges, but still too runny in the center. 
Have you tried a lid? (or bowl, if on a flat-top griddle) Trapping steam on top of the egg helps to cook it more evenly, IME (not that I claim to be all that good at it.) And it's not just me, as I've seen actual fry cooks use the bowl trick. As for judging done-ness, that's a matter of "how jiggly is the yolk" but despite your title, your question seems to be more about how to achieve it, rather than how to judge it. 