The best way when learning to cook is the thermometer. Just be sure to get the point into the center of the meat and not touching bone or near an edge.
Raw meat is soft. As it cooks, it becomes firmer to the touch. So rare is soft, medium is firmer and well done firm. With practice, you will eventually learn to cook this way.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan