If you read the last page of Part Two ("The Sieve and the Sand") of Ray Bradbury's Fahrenheit 451, you will find your answer. At the end of this chapter, Montag has been at the firehouse and listening as Beatty tries to push and bait him in one ear, while Faber (through a device in Montag's ear) plays the role of reason, explaining what Montag must do. Faber says...
All right, he's had his say. You must take it in. I'll say my say, too, in the next few hours. And you'll take it in. And you'll try to judge them and make your decisions as to which way to jump, or fall. But I want it to be your decision, not mine and not the Captain's...And it's up to you to know with which ear you'll listen.
With this in mind, the station bell rings and the men scramble to their places on the Salamander (the truck). But this time, Beatty (the Captain) is driving, which Montag notes is very unusual. And he's yelling things to Montag as he wildly weaves through the streets. There is the foreshadowing of evil in Montag's description of Beatty.
Beatty never drove, but he was driving tonight, slamming the Salamander around corners, leaning forward high on the driver's throne, his massive black slicker flapping out behind so that he seemed a great black bat flying above the engine...
With all that Beatty said to Montag at the firehouse, and Faber's words so recently ringing in his ear, the Salamander "boomed to a halt," and Montag is terribly preoccupied. He does not know how he can burn another house. He has made a decision and he is torn apart in that he knows what he believes to be right, but also knows the danger of letting on how he feels. What excuse can he make to fore-go participating in the burning of another house without giving himself away?
I can't do it, he thought. How can I go at this new assignment, how can I go on burning things? I can't go in this place...
At last Montag raised his eyes and turned.
Beatty was watching his face.
"Something the matter, Montag?"
"Why," said Montag slowly, "we've stopped in front of my house."
Beatty knows Montag's secret already. It is safe to assume that he knew while they spoke at the firehouse, for Beatty chooses to drive the Salamander, acting uncharacteristically crazy on the ride. And in the next chapter, we find that Mildred, Montag's wife, has turned Montag in, but the reader may (with good reason) feel that Beatty had his suspicions all along—long before Mildred made her choice!