How does Atticus prove that Ewell is the one that hit Mayella?

By focusing on the side of Mayella's face that was beaten up (the right) which would have resulted in Tom using his left hand; however Tom could not use his left hand as it was crippled; Bob Ewell is left-handed so therefore the evidence is clear that it was Bob Ewell who beat Mayella up. Also, Bob never called a doctor which is strange considering her physically condition.