How to tell which one?

We scopp litter boxes twice a day, this morning I notice that there is a poo that is not well formed and has hair in it. I will be watching them even closer now - but with 4 cats - how can you tell which one it is? Short of following them each to the litter box or locking them in their own rooms.... how to tell?

I would key on the one that the other cats are treating differently. The sickly one might be acting normal to your eyes, but the other cats will notice something is different before you will. This is only a suggestion and others might have better ideas. Hope it works out!