Other than the statements he makes about what he sees, which turn out to be lies, the only evidence is that Bob Ewell is left handed. When Atticus asks Bob whether he agrees with everything Heck Tate said on the stand about the condition of Mayella's face, Bob agreed. She had all of her bruises and a blackened eye on her right side. That meant that the person who beat her had to lead exclusively with his left hand. Atticus has Bob sign his name to show which hand he favored. Signing his name, Bob proved to the judge/jury that he was left handed and could have easily beaten his own daughter. At this point, we don't know if Tom is left handed or not. That comes in the next 2 chapters. So the evidence proves that Bob could be guilty of beating her as well.
Ewell signs his guilt and Tom's innocence when he writes with his left hand.
In spite of such flagrant proof, Tom Robinson has been already condemned as "guilty" simply because he is black.
It does not seem to matter that an innocent man is to be convicted and condemned for a crime he obviously didn't commit. What matters is the white community's solidarity at "saving face" when a white woman's reputation is at stake, for it is better to defend her interests than those of a Negro, a second-class citizen.
The absurdity of such a mock trial and the hypocrisy of the verdict (in light of such proof) portray the injustice of the whole legal system of the time.
When Atticus unveils that this is indeed the crime of Bob Ewell, Mayella breaks down because she is caught and possibly the only reason she is testifying is because she is scared of what her father would do to her. Atticus proves that Bob is left handed and the only person who could have committed the crime is left handed meaning Bob Ewell.