Check both the front and the back of the photograph (and its case if it has one) for a photographer's name or imprint. If you're lucky, the photographer's imprint will also list the location of his studio. Check city directories for the area (found in libraries) or ask the members of local historical or genealogical society to determine the time period the photographer was in business. You may also be able to find a published directory of photographers working in your specific region, such as
Directory of Pennsylvania Photographers, 1839-1900 by Linda A. Ries and Jay W. Ruby (Pennsylvania Historical and Museum Commission, 1999) or this online list of
Early St. Louis Photographers maintained by David A. Lossos. Some photographers were only in business for a few years, so this information may help you really narrow down the time period when a photograph was taken.