Things You'll Need
Instructions
Scour the photograph for context clues pertaining to time or place. A magnifying glass can come in handy while you search for minor, negligible details. Look for newspapers in the picture, calendars or signs of extreme weather conditions that could be used to approximate the time period or location. Note if any weaponry or insignia is in clear focus---it may be identifiable and thereby able to be dated.
Look for anachronisms---things that are out of place for the time period. If the photo has a landscape in the background, look for evidence of modern inventions that did not exist in the Civil War era.
Check to see if the photo bears a photographer's imprint. This can be as simple as the photographer's name and it can be invaluable. Should you find a name, investigate this person on the Internet or with a local historical society. Also look for any stamps on the image---during the time period of the Civil War, Congress passed an act requiring photographers to "stamp" photographs being sold. This tax lasted from August 1864 to August 1866, so only the most faithful recreation would include it. However, don't discount a photograph simply because it does not have a stamp.
Determine the type of photograph it is. Certain types of production methods were used during the Civil War period. Tintypes, for example, were popular during the Civil War. Their affordability enabled soldiers to send photographs of themselves in army regalia home to their families. One way to identify a tintype is that it will attract a small magnet. Ambrotypes and daguerreotypes were also in use. Check out http://www.billblanton.com/date.htm in Resources for descriptions of these particular products.
Consult an expert. The Center for Civil War Photography (see Resources) is also an excellent source for help.