Pull it apart and see what color fur is in it.
Of course I'm not sure this would work with a dry fed cat, not sure how much fur they pass. Went down to scoop because I smelled poop and there was a double piece on the mat. Pulled it apart and saw it was mostly Mazy cat's fur so I knew it was hers.
Actually this would work for identifying poop in the litter box too.
Of course it also depends on how many cats you have and their fur color. With my three, each of them has a distinct fur color that shows up in combings and poop. Mazy's is very light, Jennie's is darker and Queen Eva's is darkest.