Look at the Color
Perhaps the most straightforward approach is to have a look at the color of the meat inside the steak. On the underside of the steak (the side that will face the plate), cut a little slit down to the center of the meat and peek inside. When it's cooked just the way you want it, turn the steak over and press the surface with your fingertip. Note how it feels so that next time you won't need to cut into your steak.