The firemen only know which houses have books in them if someone tells. They don't have any detectors or anything that let them find the books for themselves.
For example, there was the old lady with all the books -- the one who burned herself up along with her books when the firemen came. The firemen knew about her house because one of her neighbors told on her. And Montag's house was the same way. Both Millie Montag herself and one of her friends told on Guy.
As for the hound, someone else just asked that question. I'll include a link. You can find more of a physical description of the hound early in the book -- the first time guy goes to the firehouse.