Scout has one revelation about the Tom Robinson case just as she is entering the courthouse prior to the start of the trial in Chapter 16.

As Scout, Jem, and Dill make their way into the courthouse with the rest of the spectating crowd, Scout gets separated from the boys while trying to make her way to the staircase. In her moment of separation, Scout "found herself in the middle of the Idlers' Club"; the "Idlers' Club is Maycomb's name for a group of elderly men "who had spent their lives doing nothing and passed their twilight days doing the same on pine benches" in the town square. As frequenters of the town square, they are also frequent spectators of the courtroom, who often express their opinions about courtroom proceedings.

While in the midst of the Idlers' Club, Scout overhears them having an interesting conversation about her father. One man expresses his objection to Atticus's intentions to truly defend Tom Robinson, whereas a second man responds by saying, "Lemme tell you somethin' now, Billy ... you know the court appointed him to defend this nigger" (Ch. 16). The news that Atticus was appointed by the court to defend Robinson hit home with Scout. Up to this point, she had been wondering why Atticus had been so willing to face so much ridicule just to defend a "nigger," and the man's comment helps Scout understand at least some of Atticus's moral obligation, as she expresses in the following:

This was news, news that put a different light on things: Atticus had to, whether he wanted to or not. I thought it odd that he hadn't said anything to us about it—we could have used it many times in defending him and ourselves. He had to, that's why he was doing it, equaled fewer fights and less fussing. (Ch. 16)

In other words, Scout has learned for the first time that Atticus was commanded by the judge to defend Tom Robinson. Therefore, she has come to understand part of Atticus's moral obligation to defend Robinson—he was doing so because he was commanded to do so. The understanding helps eliminate at least some of the confusion that had been weighing on Scout's mind since Chapter 9. Since that chapter, Scout has been wondering why Atticus is so willing to face so much persecution. Yet, being young, she is only able to see a small part of his moral obligation. Scout still fails to understand that Atticus has to defend Robinson not so much because he was commanded to by the judge but because he has an ethical responsibility to adhere to the principle of "innocent until proven guilty" and give every defendant the best defense possible, regardless of race.

What information about the trial in To Kill a Mockingbird does Scout discover that she did not know before?

There is actually some seemingly contradictory evidence concerning how Scout first learned that Atticus is appointed to defend Tom Robinson at his trial in To Kill a Mockingbird. In Chapter 9, Scout is deliberately eavesdropping on Atticus' conversation with Uncle Jack. Atticus tells Jack that

"... John Taylor pointed at me and said ' You're It.' "

Apparently Scout does not understand this reference that Judge Taylor has assigned Atticus to the case, rather than Atticus seeking it out on his own. In Chapter 16, however, Jem, Scout and Dill all sneak down to the courthouse where they overhear several men discussing the trial. When they hear Atticus' name mentioned, they pay closer attention. Scout learns that

     "... you know the court appointed him to defend this nigger."
     "Yeah, but Atticus aims to defend him. That's what I don't like about it.
     This was news, news that put a different light on things...

This is Scout's first inkling that her father was appointed to the case, despite what she had overheard at Finch's Landing.