Here are three approaches to finding historical maps, roughly defined as pre-20th century.
1. Limit by publication year
2. Identify appropriate subject headings
Library of Congress Subject Headings may help in identifying older maps. You can browse subject headings in Mirlyn by typing a keyword and selecting the 'subject' button. The LCSH volumes may also be consulted.
Some useful LCSH headings for historic maps:
3. Historical Atlases
Historical Atlases include maps and are important supplements in answering research questions. Depending on the location and the time period, it might be the only item that fits your particular query. Search for historical atlases by keyword, or using the subject headings listed above.