How does the author use foreshadowing in School for Scandal?

In Act I, Scene 1, Joseph Surface says that Snake is such a villain that he cannot even stay true to his own villainy, foreshadowing his willingness to betray Sneerwell and Joseph later in the play. Because Snake is so willing to sell his services, Joseph sees that he will sell off the people who hired him if the money is right and will have no compunctions about leaving others holding the bag.