The fact that Tom has a lame left arm is found in Chapter 18. This fact is revealed in the way that Atticus gets Maybella to positively identify the defendant. She says she is certain it was Tom who grabbed, choked and raped her. Atticus asks Tom to stand:
...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 from the balcony I (Scout) could see that it was no use to him.
The implication, of course, is that Tom would have needed two fully functioning arms to commit the crimes he was accused of perpetrating.
Mayella lied about Tom Robinson's rape. Atticus asked Tom to stand up, which revealed his non-functioning arm. To commit a crime like that, he would need two fully functioning arms to do that.