Here is a demonstration that was filmed in a cooking school (or a pastry/baking school? - the video doesn't state the source) of an instructor tempering the chocolate on a marble.
Chef Schorner's tip:
Feel the bottom of the bowl that the chocolate is in. There's an old saying, if you feel it warmer than your body, it's usually too warm. If it feels colder than your body, it's usually too cold.His method: