How has beatty given Montag hints that he is under suspicion?

Beatty regularly speaks of the nobility in burning books, but as time has gone on he takes a more direct approach. He argues books themselves, and the words they contain; he quotes literature, and he does all of these things for two reasons. One, to trip Montag up and get him to say something he shouldn't, and two, to let him know he's under suspicion.