What body language did Scout see that let her know the verdict even before it was announced?

Scout has a feeling that she knows the verdict before she hears it.  Right before the jury returns with the verdict, she feels the same way that she did the morning Atticus shot the mad dog.  Scout knows that Atticus is doing what needs to be done, but something unhappy or sad will come out of it.

The colored people stand out of respect for Atticus. They are appreciative that he tried his best to defend Tom Robinson and put his entire reputation on the line to see justice served.