Use a standard search engine to see if the passage is easily locatable on the web.
Search Google Books. Google has been digitizing massive numbers of public domain books and a Google Book search sometimes picks up passages not found by regular search engines.
Try Google Scholar in case the passage has been cited in scholarly literature, but not in material found elsewhere on the internet.
Upload the passage into a plagiarism detector and see if it can spot the source.
Go to a public or university library. Library computers access many scholarly sources not available to the public.
Search the general academic indices such as JStor or Gale Infotrac products.
Search subject and period specific indices. For example, for early English titles, including incunabula, consult Early English Books Online. For Church Fathers, search the Patrologia Latina or Graeca, now available from the Internet Archive.
Consult a reference librarian for recommendations of other archives to search.
Search the internet for an e-text of the book if you know the source. Download the e-text and use the search function of your word processing program to locate a specific passage
Search indices of print books to find possible locations of a passage.
Skim the book containing the passage.