Scout actually comes to several conclusions during Tom Robinson's testimony in Chapter 19 of To Kill a Mockingbird. First, she noticed that Tom's "rubber-like left hand... slipped off the Bible and hit the clerk's table." She already knew that the injuries to Mayella had to have been made by a left-handed man and, to her, this eliminated Tom as a suspect. Secondly, Scout realized that
Mayella Ewell must have been the loneliest person in the world... even lonelier than Boo Radley... Tom Robinson was probably the only person who was ever decent to her. But she said he took advantage of her, and... she looked at him as if he were dirt beneath her feet.
Scout also remembered one of Atticus' courtroom tricks: Sometimes it was better to listen to the witness rather than watch him. When she listened to Tom, she decided that he was truthful, "with no hint of whining in his voice."
He seemed to be a respectable Negro, and a respectable Negro would never go up into somebody's yard of his own volition.
Scout also noticed that Tom's manners were as good as her father's, and she later came to understand that Tom's running from the Ewell house was not a sign of guilt--just an act of self-preservation.
In Chapter 19, Tom Robinson testifies in court and tells Atticus that Mayella Ewell continually asked for his help when he passed by her home. Tom felt sympathy for Mayella and always offered her a helping hand. As Tom is testifying, Scout says she came to the realization that Mayella Ewell was the loneliest person in the world. Scout believes that Mayella was even lonelier than her reclusive neighbor, Boo Radley. After Atticus had asked Mayella if she had any friends, Mayella was confused and thought he was making fun of her. Scout begins to pity Mayella and thinks that she is as lonely as a child of more than one race. Scout reasons that Tom Robinson was the only person who treated her decently, but notices how Mayella looks at him as if he is beneath her. Scout's realization is significant because Tom's testimony that Mayella kissed him is believable given the fact that Mayella is extremely lonely and Tom was the only person who ever gave her the time of day.