I noticed that Peeka had lost weight and she was always a chubbette. I watched her that night and noticed that she didn't move out of the spot where she had lain all day in the sun. So I called the vet, they took her right away and told me that they were almost positive it was renal failure. Peeka didn't respond to any treatment, none of it. She wouldn't eat, not even cooked chicken, her absolute favorite. She was on IV fluids and still could not get hydrated. Then her eyes and the inside of her ears turned yellow which meant jaundice and that meant her liver was shutting down, too. She stayed in the hospital for 5 straight days and didn't respond to any treatment whatsoever. When she cried every time I left after visiting and I asked if I brought her home, would it be painful for her to die naturally, I was told that it would be, so it broke my heart but I had her PTS. So I guess you can say that some of the signs are that a cat goes downhill rapidly and doesn't respond to treatment. I'm sure the vet tech who posts here (I think her screen ID is Catlady711 but I'm not sure) can give you more concrete things to look for.