How does Scout know the verdict before it is read? What is the broader implication of the jury's behavior in To Kill a Mockingbird?

Being the daughter of an attorney in To Kill a
Mockingbird, Scout had a little background about juries and how they react.
She describes the return of the jury for the Tom Robinson trial as having "a dreamlike
quality... moving like underwater swimmers." She saw

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     ... something only a lawyer's child could be
expected to see, could be expected to watch for...
     A jury never looks at
a defendant it has convicted. and when the jury came in, not one of them looked at Tom
Robinson. 

It was no surprise
to her, then, when Judge Taylor polled the jury and received the repeated answers of
"guilty." Jem must have been surprised, however, because he "jerked at each guilty" as
if he was being stabbed. Scout realized later that Tom had no chance in the first place;
the all-white, all-male jury had made up its mind even before the trial had started.
For,

... in the secret courts of men's hearts
Atticus had no case. Tom was a dead man the moment Mayella Ewell opened her mouth and
screamed.