With no visual differences in the excrement of the two species it’s generally not possible to do more than make an educated guess based on the manner in which it is deposited. The only certainty will be when the creature is actually observed to make the deposit or when DNA analysis is carried out. Otherwise it can be considered as being a high likelihood of  being Stone Marten when excrement is found concentrated in a latrine either in a roof space or on elevated ledges. Both species share much the same seasonal diets of small birds and their eggs, small mammals, insects and fruits.

When fresh the scats tend to have a musty sweet scent that is not unpleasant. This is quite an important factor as other creatures that may produce similar excrement have strong pungent scents. Those with an inquisitive mind can use a stick  and take a small piece to sniff. 

This page provides some examples of Pine Marten and Stone Marten droppings in an effort to show the incredible range of forms they can be found in that result from the changing seasons and the variety of food available to them in any locality.