This is a good question. As you undoubtedly know Atticus is a lawyer. Hence, he knows the law. This comes out in many places, such as when he discusses the importance of reasonable doubt. He says to Jem that the jury should have reasonable doubt, because there was no eyewitness. This is particularly important, because rape is a capital offense. Based on this point, according to Atticus, Tom Robinson should be declared innocent. Here is the dialogue and quote:
“But lots of folks have been hung—hanged—on circumstantial evidence,” said Jem.
“I know, and lots of ‘em probably deserved it, too—but in the absence of eye- witnesses there’s always a doubt, some times only the shadow of a doubt. The law says ’reasonable doubt,‘ but I think a defendant’s entitled to the shadow of a doubt. There’s always the possibility, no matter how improbable, that he’s innocent.”
Here is another example of Atticus' knowledge of the law. After the pronouncement of guilt upon Tom Robinson, he told his son not to worry, because there will be an appeal. In other words, Atticus will appeal to an appeals court. Moreover, he believes that he had a good chance of winning there. Here is the quote.
“It’s not time to worry yet,” Atticus reassured him, as we went to the diningroom. “We’re not through yet. There’ll be an appeal, you can count on that. Gracious alive, Cal, what’s all this?” He was staring at his breakfast plate.