We first see Tom Robinson's arm at the trial after Heck Tate, Mayella and Bob have all testified. The information they reveal contrasts against Tom's testimony in many ways, but the first is the physical description of injuries and the cold hard truth that your question begs to reveal. The person who injured Mayella appears to have been able to lead strongly with their left arm.
Tom's left arm got caught in a cotton gin while he was at work on Mr. Link Deas' farm.
Another detail that deals with this arm occurs Mr. Gilmer's suggestion that Tom was a good fighter anyway and could have done to Mayella what he did even though he is crippled. That argument was disproved by Atticus.
Link Deas also stands up to testify (although he wasn't asked) on behalf of Robinson from the middle of the courtroom. He insinuated that Tom had always been a good worker for him.