Edges By the 1880s, cabinet card mounts sometimes had bevelled edges, and were often finished in gold or silver.
Colour The colour of the cardboard mount can also help date the photograph.
Other things to look for on women are the presence and size of a bustle and the fullness of the skirt.
For men, look at vests, neckties, the fit of a jacket (loose or fitted) and how it is buttoned.
Enter a photo type into its search engine and you will see many examples that may turn out to be similar to the photograph you are researching. Both the men in my photo are young, but one appears to be older than the other, and he has arm slung around his younger brother's shoulders. The elder is wearing a watch chain and a pinky ring. Michael was seven years older than Peter, who was seven years older than Timothy.