How does the reader know that Boo Radley saw the children coming?

When Jem and Scout are standing in the street, Scout is so transfixed by what's going on that she doesn't notice a blanket being put around her. Since they're standing by the Radley Place and Nathan Radley is the type of man to bound out the house waving a gun around, it can be deduced to absolute certainty that Boo Radley was the one to exit the house and place the blanket around Scout.