Thematic catalogs can be used to locate vocal music by a specific composer. Most of these are organized by opus or genre, and they often include an incipit (the first few bars of each movement) to help you identify the piece you're trying to find.
For more information on thematic catalogs, please see Barry Brook's Thematic Catalogue entry in Grove Music Online.
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The text and examples on this page are courtesy of Bowling Green State University Library.