Go to your local library and peruse their catalog system. Most libraries have computer terminals that allow you look up the book in question using the system. Keep in mind that the specific book's number may vary between libraries. The numbers and the corresponding classes are:
000 -- General works (including computer science, information and miscellaneous)
100 -- Philosophy (and psychology)
200 -- Religion
300 -- Social sciences
400 -- Languages
500 -- Pure sciences (including mathematics)
600 -- Technology (practical arts, including applied science, medicine, engineering, business accounting, agriculture and salesmanship)
700 -- Fine arts (and recreation, including architecture, painting, photography, music, and other amusements)
800 -- Literature
900 -- History, geography and biography
Use the number that you obtained from the library's computer system to determine what section of the library your book will be located in.
Physically look for the book on the library's shelf. The Dewey system will direct you to the specific section, but you will have to hunt for it on your own.