In general, CDVs which date to the Civil War era of 1861 to 1865 can be determined by the weight of the card stock upon which the photograph is mounted, or by the backmark of the card.  In addition, if you are familiar with certain fashion styles which came around near the end of the war, you can be fairly certain that the picture dates from that time frame.
 

• Civil War era CDVs often have a lighter weight cardstock than those of post-war, and are 4 1/4 by 2/1/2 inches in size.

• Often there is a single or double colored border near the edge of the cardstock front, to frame the picture.

• Backmarks for Civil War era CDVs advertised the name and location of the photographer.  They were usually printed small, horizontally centered across the back of the card, and did not take up much more than an inch and a half (click here for an example).  They did not the have the ornate, vertical lettering and beautiful graphic designs that you see on post-war CDVs.

•  Civil War CDVs with revenue stamps on the back can be dated to the period from August 1, 1864 to August 1, 1866.  The Revenue Act was passed by Congress in an effort to raise money for the war, and was put on various documents including photographs.  An amendment allowing for 1-cent stamps for very cheap cards was begun in March 1865, so those with 1-cent stamps can be dated with accuracy between March 1865 to August 1866.  Blue stamps can be dated to the summer of 1866, and are obviously post-war.

Dating Old Photographs
Dating Portraits- Tax Stamp