In the chapters relaying the trial scenes of Harper Lee's To Kill a Mockingbird, Atticus's cross-examination of the witnesses reveals several points of circumstantial evidence that point to Tom Robinson's innocence, not his guilt.
The first piece of crucial evidence is revealed during Sheriff Heck Tate's testimony, who confirms that Mayella Ewell had been bruised on the right side of her face. This proves to be a crucial point because Mayella also testifies that she had been attacked from the front, which means that only a man who could use his left hand could have hit her on the right side of her face while facing her. Heck Tate's second point of crucial evidence is that Mayella had been strangled by a man gripping her neck with both hands, which was evident due to the fact that she had bruises all around her neck. Again, only a man capable of using both hands could have bruised her entire neck.
Atticus's cross-examination of Bob Ewell, Mayella's father, is even more revealing. While on the witness stand, Atticus asks Ewell if he is literate, and when Ewell answers in the affirmative, Atticus asks him to write his name. Interestingly, Ewell writes before the court using his left hand. When asked by Judge Taylor if he is ambidextrous, Ewell responds with the very intelligent answer, "I most positively am not, I can use one hand good as the other. One hand good as the other," which shows that Ewell was very capable of having bruised Mayella on the right side of her face and of having strangled her with both hands (Ch. 17).
The most irrefutable evidence is revealed during Mayella's cross-examination. At one point, Atticus asks Mayella to point out the man in the courtroom she is charging with having attacked her. Mayella points to Tom Robinson, and Atticus has Robinson stand before the court. As he stands, he reveals that, as Jem points out, "he's crippled!" (Ch. 18). Scout further describes in her narration that Robinson's left arm was a whole foot shorter than his right and "hung dead at his side" (Ch. 18). His left hand was also shriveled and appeared to be completely useless. In fact, his left arm and hand are so useless, Robinson finds it impossible to keep his left hand on the Bible long enough for him to be sworn into the witness stand. Reverend Sykes explains to the children that Robinson had caught his left arm in Dolphus Raymond's cotton gin when he was a boy and nearly "bled to death" (Ch. 18). Robinson's physical status serves as undeniable proof it would have been impossible for him to have bruised Mayella on the right side of her face or to strangle her, which makes him look completely innocent of all he is being accused of.