The trial of Tom Robinson begins in Chapter 17, and Atticus gets right to work trying to show that it could not have been Tom who beat and raped Mayella. First, Atticus determines from Sheriff Tate that no medical evidence existed and that Mayella never saw a doctor. This brought into question whether a rape even occurred. Next, Atticus determines that Mayella was beaten on the right side of her face and that her neck had marks "all around her throat."
Bob Ewell took the stand next. Atticus requested that Ewell sign his name, and he did so with his left hand. Scout recognized what her father was up to: Mayella must have been beaten not only by a left-handed man, but by someone with two good, strong hands in order to make the marks around her neck. Bob Ewell could have been that mind, Scout decided.