Here is a link to a review of a book published by Family Chronicle titled Dating Old Photographs. I'm giving you this link because it describes a very helpful book and it may entice you to want to look at it. The review also suggests that a good way to use the book it to compare your own photographs with its 700 or so pictures covering the period from the 1840s to the 1920s. You will be able to compare clothing and hair fashion, the poses adopted by the subject and the background settings...all of which could help you date your photograph.
If the review strikes your fancy, I have good news. We have two copies of the book in our collection at the Largo Public Library, and at least one of those is circulating. And even better, there is follow-on book titled More Dating Old Photographs that we also have. The books have the call number 929.1028.
Check out the review to get a better idea of the book at http://www.familychronicle.com/datphoto.htm