The location of the stain, how the stain smells, and the color of the stain may be able to offer you some indicators as to what it's from. For instance, old oil or grease stains may smell a little rancid, but appear dry. If the color and scents correspond with a potential stain source such as wine, chocolate, or fruit, then you will have more information to go off of.
As a rule of thumb, if a stain appears brown or rich in color, it is protein-based and will require a stain treatment with enzymes to lift it. If the stain is gray, it is rich in oil or grease and will require a stain remover treated to latch onto oil molecules to dissolve it. If the spot is yellow, it is either a sweat stain or a combination stain, consisting of both protein and oil.