How does Atticus show that Tom Robinson could not have committed the crime he is accused of ?

Atticus questions Heck Tate, asking whether anyone called a doctor. Mr. Tate says no. Atticus asks where Mayella had been beaten, and Mr. Tate says, with some hesitation, that her right eye and entire right side of her face were bruised, and she had scratches all around her neck. A little while later while questioning Mr. Ewell, Atticus asks if he can write, and he says he can, so Atticus asks him to write his name on an envelope. In so doing, it is revealed that Mr. Ewell is left-handed. 

When it comes time to question Mayella, Atticus asks her to identify the man who raped her, and Mayella points to Tom, who Atticus asks to stand. Everyone in the courtroom notices that Tom's left arm is twelve inches shorter than his right, due to an accident in his youth when the arm got stuck a cotton gin. Tom Robinson could not have been the assailant.