This can be a great help, especially if you check while the patron is still at the library. For example, if you are asked for the address of the ABC Company in Aurora, Illinois, you can check that there really is such a company there. The patron might mean Aurora, Missouri, or the company might be out of business. This kind of checking can give you a clue so you can question the patron further and more intelligently while she is still at the library. It is a good idea to be able to instruct patrons on how they can use the directory assistance service themselves to find this kind of information.
Instructions are in the front of the telephone book.