The reader of the novel will have to decide for him/herself. Author Harper Lee never reveals the truth about what really happens that night, only allowing Scout's narrative to detail the stories of those involved. Most readers will believe what Atticus, Scout and Jem conclude: that Tom's version of...
the events are true, and that the Ewells are lying. According to Bob, he comes home to find Tom "ruttin' on my Mayella"--beating and raping her; Tom then runs off. Mayella's own story is confusing: She never details the supposed rape, and she goes back and forth about whether Tom actually assaults her.
"No, I don't recollect if he hit me. I mean yes I do, he hit me." "Was your last sentence correct?" "Huh? Yes he hit--I just don't remember... it all happened so quick." (Chapter 18)
Tom claims that he is invited into the Ewell house to fix a loose door, but he is then attacked inside by Mayella, who hugs and kisses him. According to Tom, Mayella has sent the other Ewell children to buy ice cream--" '...took me a slap year to save sebm nickels... They all gone to town.' "--presumably to have the house to herself with Tom. Tom says that she tells him
"... she 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." (Chapter 19)
Tom admits that he ran when Bob showed up, but claims he did not "harm her in any way." The only thing he hears as he flees is Bob threatening Mayella:
"... you goddam whore, I'll kill ya!" (Chapter 19)
Sheriff Tate's testimony is probably truthful, but it is primarily Bob's version of the story. Tate admits that Mayella is "pretty well beat up" and still "lying on the floor" when he arrives at the Ewell house. Neither he nor Bob requested medical attention for Mayella.
What really happened? Tom's story is probably true, and it is Bob who beats his own daughter for having "Tempted a Negro." Bob decides to blame Tom for his own assault, and the frightened Mayella goes along with his story. Did a rape occur? Probably not, since Mayella is clothed when Sheriff Tate arrives. But Mayella's own words about "what her papa do don't count" and the nodding of her head when Atticus asks if Bob isn't "tollable" when he's been drinking leads one to believe that Bob may have taken advantage of Mayella before.