Often someone will put a photograph in front of you and ask you to give them some information on it. Some obvious ways to date the photograph is hair style, dress, objects in the photograph like a car, airplane, etc. All of these things call for a study of history of costumes and dress, etc. An interesting article by Jen Thompson on the Hooverette dress is at this web site
It is an interesting blog on sewing clothing and I recommend you take a look.