The better way to date the photograph is by the paper and the type of processing. Very early prints were the size of the negative and the way the paper was processed took many forms. It wasn't till later that enlargers were used and the printing became a 2 stage chemical process using silver nitrate papers. As well the name of the company who produced the picture was usually stamped on the back of the picture. The following link may give you a better idea.
http://www.bl.uk/onlinegallery/features/photographicproject/photographicprocesses.html
