Find the book's title, author's name and the table of contents. Some books are published anonymously, but author's names are often released shortly after publication. The table of contents usually will help identify the book. For example, a 1667 edition of "Paradise Lost" has 10 "books" (or chapters), and the 1674 edition has 12.
Look closely at the quality of the book's condition. There are a number of forgeries of rare books that date back to the late 18th and early 19th centuries. Usually, when the paper quality and binding of a 15th century book is in really great shape, you are encountering a forgery.
Review any distinctive characteristics of the book, since differences in production help narrow the time line of the book. For instance, woodcut images were frequently used in the 15th and 16th centuries, but engraved images took off in the 19th century.
Search for book stamps or markers of possession. Many collectors or owners have an "Ex Libris" stamp with the full name of the owner, or sign their names in prominent places. Also, find any handwriting or other markings. Historical and personal differences in methods of shorthand and even handwriting style can provide a lead in your investigation.
Submit any phrases or images you have into searchable databases, such as Early English Books Online (EEBO). Such databases should be available at the libraries of local colleges. Also, request the attention of rare book librarians at reputable institutions, such as Yale University's Beinecke Library (in New Haven, Connecticut) or the Huntington Library (in San Marino, California).
Compare the book to any other copies that are out there. Differences might shed more light on the publication history or the circumstances of circulation. Often, authors would publish special "presentation copies" for their supporters, and these copies would have special content.