In To Kill a Mockingbird, how does the reader learn that Mayella Ewell is lying during the trial?

In To Kill a Mockingbird, Mayella Ewell is the victim of a savage beating. She tells Sheriff Tate that Tom Robinson, an African American man, is the one responsible. During the trial, Mayella’s testimony, combined with other evidence, proves that she is lying, and more importantly, that Tom Robinson is innocent.

At the trial, Mayella testifies under oath that Tom Robinson attempted to strangle her with both hands and punched her in the face. Initially, this story supports Sheriff Tate’s testimony that Mayella had a black right eye and choke marks around her throat shortly after the crime occurred. Yet under cross examination, Atticus shows the jury that Tom’s left arm is crippled due to a childhood accident. There is no way he could have thrown a punch with his left hand, or put both hands around her throat. Despite the glaring contractions between the evidence and Mayella’s testimony, the all-white jury finds Tom Robinson guilty.