Atticus knows before the trial even begins that the outcome will boil down to Tom Robinson's word against the Ewells'. Tom has been charged with both assault and rape, but Bob Ewell is the only "witness" to the attack, reporting to Sheriff Heck Tate that "some nigger'd raped his girl." When Sheriff Tate went to the Ewell house, he asked Mayella if Tom
"... beat her like that, she said yes he had. Asked her if he took advantage of her, and she said yes he had."
Tate then arrested Tom on Mayella's word alone. On cross-examination, Atticus repeatedly asked the sheriff why he had not called a doctor. Tate replied that he could see for himself that Mayella "was mighty banged up." But Tate could not tell if Mayella had been raped, and only a doctor could tell for sure. No doctor was ever called to examine Mayella for evidence of semen, hair or bodily fluids, which would have determined if a rape had occurred. (Bob later admits that he wasn't about to waste five dollars for a doctor's bill.) Atticus's question is also meant to show to the jury that there was no medical evidence a rape had occurred in the hope that doubt would be established about Bob's and Mayella's story.