The process for these pictures was done by using a plate, made of copper and using silver iodide to expose the picture, along with exposure to light.
Tin type pictures were introduced in 1853, and became extremely popular.
There are pictures that used daguerreotype, glass plate, cabinet card, and the thickness of the cabinet card can also tell the age of the picture.
Looking at a picture and dating it, takes noticing all of the clues possible.
Men, on the other hand can be difficult as suits did not change from year to year.