In Chapter 19 of To Kill a Mockingbird, what does Scout realize about Tom Robinson's testimony?
Scout takes her father's advice and listens to Tom's testimony as opposed to watching him in order to tell if he is lying. When Atticus asks Tom if he ever entered Mayella Ewell's yard without an invitation, Tom says, "No suh, Mr. Finch, I never did. I wouldn't do that, suh" (Lee, 118). Despite protesting too much by denying it three times, Scout finds herself believing Tom. Scout feels like Tom is telling the truth because he seems like a respectable man.
After Tom testifies that he resisted Mayella's advances and ran out of the house, Scout mentions that it occurred to her that Tom's manners were as a good as Atticus's. Later on, Atticus explains Tom's predicament to his daughter. When Mr. Gilmer asks Tom why he helped Mayella without receiving any monetary compensation for his work, Tom replies that he felt sorry for her. Scout realizes immediately that nobody liked Tom's answer. Overall, Scout believes Tom Robinson and does not think he assaulted or raped Mayella. Scout also notices that the jury was not pleased after Tom said that he felt sorry for Mayella. Unfortunately, Scout does not get to hear the end of Mr. Gilmer's cross-examination because Dill begins to cry.
One piece of advice Scout remembered from her talks with Atticus was that sometimes it was
... better to listen rather than watch: I applied this test.
Scout determined that Tom was not lying about setting foot on the Ewell property without an invitation. She also decided that no "respectable Negro" would walk onto a white man's property without permission. She also realized that Tom's good manners were part of his predicament. Even though he had not touched Mayella (she had hugged and kissed him), he knew that the accusation against him--that he had struck Mayella--would mean that he would not live long,
... so he took the first opportunity to run--a sure sign of guilt.