The answer to this question depends on three quotes regarding three different witnesses at the trial.
First, Heck Tate testifies about Mayella's injuries:
Something had been made plain to Atticus also, and it brought him to his feet. “Sheriff, please repeat what you said.”
“It was her right eye, I said.”
“No…” Atticus walked to the court reporter’s desk and bent down to the furiously scribbling hand. It stopped, flipped back the shorthand pad, and the court reporter said, “‘Mr. Finch. I remember now she was banged up on that side of the face.’”
Atticus looked up at Mr. Tate. “Which side again, Heck?”
“The right side, Mr. Finch, but she had more bruises—you wanta hear about ‘em?”
Atticus seemed to be bordering on another question, but he thought better of it and said, “Yes, what were her other injuries?” As Mr. Tate answered, Atticus turned and looked at Tom Robinson as if to say this was something they hadn’t bargained for.
“…her arms were bruised, and she showed me her neck. There were definite finger marks on her gullet—”
“All around her throat? At the back of her neck?”
“I’d say they were all around, Mr. Finch.”
“Yes sir, she had a small throat..."
Through this testimony, we know first that Mayella's injuries were inflicted to the right side of her face, indicating that a left-handed person had thrown those punches. We also learn that she had marks "all around" her throat, indicating someone had tried to choke her.
Then we learn something interesting about Mayella's father at the end of his testimony:
Mr. Ewell wrote on the back of the envelope and looked up complacently to see Judge Taylor staring at him as if he were some fragrant gardenia in full bloom on the witness stand, to see Mr. Gilmer half-sitting, half-standing at his table. The jury was watching him, one man was leaning forward with his hands over the railing.
“What’s so interestin‘?” he asked.
“You’re left-handed, Mr. Ewell,” said Judge Taylor. Mr. Ewell turned angrily to the judge and said he didn’t see what his being left-handed had to do with it.
This raises suspicion about Mr. Ewell's involvement and sheds further doubt on his credibility. The fact that he is left-handed makes him a possible suspect for the violence against Mayella.
This possibility is solidified during Mayella's testimony. Atticus asks Mayella to identify the man who assaulted her, and she points to Tom. An interesting development occurs at this point:
Tom Robinson’s powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him.
Tom could not have inflicted the bruises on the right side of Mayella's face because he cannot use his left arm. Further, he could not have left marks of choking her "all around" her neck because he lacks one hand that could have made that full circle.
Therefore, the cumulative evidence points not to Tom as the man who assaulted Mayella, but to her own father, a left-handed and exceedingly violent man.