What evidence shows tom is guilty?

Tom couldn't have done it because Mr. Heck Tate said her injuries were to her right side, which would have meant that the person would of had to of been left handed. And Tom's left hand is mutilated from a cotton picking accident. Atticus had Mr. Bob Ewell sign his name and he signed it with his left hand. So, Mr. Bob Ewell had to of done it.