In Harper Lee's To Kill a Mockingbird, evidence in the Tom Robinson trial clearly show that Tom Robinson is innocent and that Bob Ewell is truly the guilty party.
Bob Ewell is the first to give evidence during Atticus's cross-examination that sheds light on his guilt. During his testimony, Atticus asks Ewell to write something for the court, and all note that Ewell writes with his left hand. Ewell then testifies that he is ambidextrous, or as Ewell intelligently states in answer to the prosecuting attorney's question if he is ambidextrous, "I most positively am not, I can use one hand good as the other" (Ch. 17). Prior to Ewell's demonstration of his left-handedness, both he and Sheriff Heck Tate had testified that the right side of Mayella's face was bruised, which could have only been accomplished by a left-handed man when facing her. Hence, Ewell's left-handedness throws suspicion on Ewell.
Furthermore, during Mayella's testimony, Atticus has Mayella point to the man who raped her. When Mayella points to Robinson, Atticus has Robinson stand before the court. The court can see that Robinson's left arm is much shorter than his right and that his left had is shriveled. As Scout notes in her narrative, Robinson's left arm "hung dead at this side" (Ch. 18). Reverend Sykes notes that Robinson's left arm was crippled when he got it caught in a cotton gin as a young boy. Since Robinson's arm is crippled and useless, it's obvious that Robinson could not possibly have been the person to bruise Mayella on the right side of her face; her father is the more likely suspect.
In addition, through Atticus's questioning, Robinson's testimony reveals some interesting things about Mayella and her family. Through Robinson's testimony, it becomes evident that Mayella lives a very lonely life and has frequently tried to flirt with Robinson. It's further evident that Mayella tried to seduce Robinson on the day in question. Robinson even testifies that she kissed him on the side of the face and said, "[S]he never kissed a grown man before an' she might as well kiss a nigger. She says what her papa do to her don't count" (Ch. 19). Interestingly, the Ewell household is full of seven young children, yet Mayella's mother has been deceased for longer than Mayella can remember, a fact that Atticus also brought to the court's attention. Hence, Robinson's testimony coupled with the seven suspicious young children in the Ewell household prove that Bob Ewell is guilty of multiple accounts of abuse, not Robinson.