How did firemen know which house had books?

Someone told Captain Beatty if they had a suspicion about someone having books or if they witnessed it with their own eyes.
Citizens sent in alarms stating addresses of places suspected of harboring literature.
because someone (like a neighbor) would alert them. then they (firemen) would check it out and see if there were really any books, then burn them if there were.
Citizens sent in alarms stating addresses of places suspected of harboring literature. The alarm is basically faxed to the fire station and the firemen follow the alarm then do what they are hired to do.
The firemen would send a mechanical hound all around town, and if the mechanical hound detected any books, then the hound would just stray around that single house, and that probably reveals to the firemen that the house is in possession of books.