I did something similar to what westtexas is saying. I had a list of things that made my Chelsea-cat who she was - she purred constantly, she meticulously groomed herself (she was all white and darn it she kept that fur white!), she cuddled with me the entire night in bed. A few other things. When those things started to fade, I knew she was nearing the end of her 6+ yr battle with renal failure. One night when she was laying in bed with me, not purring, and then piddled herself in the bed, I knew it was her time. We spent the next whole day laying around on a blanket in the yard, and then went to the vet. It was terribly hard, and I'm crying as I write this even though it's been 6 yrs, but I know it was her time and as my vet told me, I let her go on her terms.

Hugs - I know what a tough time it is.