I read about this but have never tried it (not sure if it actually works).
If you can make sure that all the cats eat and drink from separate bowls you
can put food coloring in to their water or food and it will not harm them
but it will come through in their urine. Just use a different color for each
cat.

It is easy to find out but it might take you a while to know.  Kill one cat
a week and when there is no more pee about, you should be able to figure out
which one was the culprit. :-)    With all kidding aside, it sounds like one
of your cats does not like to use the same litter box with one of the
others.  That can be a very hard one to figure out.  With eight cats, surely
you have more than one box ...if not, that indeed is the problem.  With that
many cats, you should have at the minimum of THREE litter boxes in DIFFERENT
parts of the house.  I know! ...use to have the same problem with our FIVE
cats but no more.