In multi-cat homes, when one cat urinates outside the box it can be difficult to tell which one it is. Inappropriate elimination ("peeing outside the litter box") is a big problem for homes with more than one cat.
There is no easy way to identify which cat is the culprit, but here are some methods commonly recommended by veterinarians.
1. Isolate different cats to separate areas and litter boxes. You must be careful with this method, as some cats are upset when their litter box is moved. For example, a cat who is prone to inappropriate elimination might use their box perfectly when housed separately, but urinate outside the box otherwise.
2. A nontoxic, pet-safe dye called fluorescein stain can be used with one cat to identify urine spots. This common dye, used to diagnose ulcers on the cornea of the eye, turns urine bright yellow or green when ingested. When given to one cat out of a household, it can be used to identify the "guilty cat" once urination occurs. (Note that this method is less effective in homes with more than 2 cats.)
3. Is your cat urinating in one particular area? Keep a close eye on that location and see if you notice any unusual behavior. Some cats who urinate in one inappropriate location tend to frequent that place at other times.
I hope these tips help you figure out which cat is urinating outside the litter box in your home.