I have always found it hard to tell when My Cats are mourning, although I think there are subtle signs.
I knew that My Precious missed her Sam as she was far more friendly and was always coming to me for pets , as she was not too close to the other Cats, even the Siamese without her Shemp there!
I know that Joseph misses his Moose and John Hancock as he is starting to be a solo Cat a lot more , sitting on his own.
Maybe one day if Missy becomes too lonely a Cat will come by needing a Furr Ever Home and Missy will have a New Cat Companion.