How can I determine which of my cats is doing this?

Yes, the cat person needs help.

I have 5 cats. 1 is getting older. She is 14. 3 are 10 and 1 is still young. So, a cat peed on my couch. This is the very first time that any cat has peed anywhere outside the box. Whichever cat peed on my couch is unknown. We moved some litter boxes around and removed the old couch and there was no more pee for a month. We just put blankets on the floor and sat on chairs. Got a new couch this weekend. Peed on again.

I am very upset. I have no way to figure out which cat is doing the deed. I took my eldest cat to the vet and she checked out a ok.

How can I tell which cat is doing this so I can correct the behavior? Help!