There is no sure way to tell a wolf from a dog, neither by genetic nor by appearence. The two animals fall in a same biological species. They can cross breed and produce fertile offsprings. In that sense, the words "dog" and "wolf" more have a subjective conotation of certain characters. For example, one may say there is wolf in his dog, or there is a dog in that wolf. I would think the difference between these two animals are like that between Cantonese and Beijingese - there is definitely difference, but there is definite broad ovarlap too.