One way to help date old photographs is to look at the clothing the subjects are wearing, and if it is especially stylish, you may be able to find out when such styles were in fashion by reading Fashion History books. Of course, then, as today, many people wear clothing that might not be considered the latest fashion, but which is their best clothing, in order to take a photo. How to know if your photo is of a 19th century fashionista, or a poor person in 20 year old hand-me-downs?
First, especially if the photo is a wedding photo, you can be more certain that the person is wearing their best clothing, or clothing purchased or sewn for the day. This is true for the wedding party too. Any photo that appears to have been worn for a religious ceremony may be assumed to be the person's newest or best.
Secondly, examine any jewelry worn. Does it appear to be expensive? Metal and jeweled costume jewelry was not easy to come by until the Industrial age. What about hair ornaments or combs.
Third, look at hair... is it done in an elaborate style that might have required a hairdresser or maid to assist? Does the person appear to be well groomed? Is their hairstyle home-made ? (The bowl haircut on children)
Fourth, is the person wearing a uniform? If so does the uniform identify the country or side or a battle they may have been on? Are they posed in an official way?