If you are referring to part 1 of the book, the conversation between Beatty and Montag indicates that she knows that Montague has books at home.
Explanation:
"Farenheit 451" is a book that presents a society where books are prohibited and whenever a book is found, it is set on fire by firefighters. This decision, and prohibit reading, happened because books allow society to think and reason about everything around them. Thus, a reading society is a society that is difficult to control and manipulate. In the conversation of Beatty and Montag at the beginning of the book, he presents this concept, since Beatty presents Montag as the absence of books allows the alienation and manipulation of society to be done in a very strong way. This conversation also reveals that Montag has hidden books and that it will be difficult for anyone to suspect him, since he is a firefighter.
