One of the first physical signs is drinking more water and peeing outside of the litterbox or volume of pee increased in the box and less concentrated.
Increased urination, increased drinking for starters. Followed by weight loss and lack of appetite as well. Eventually they won't eat anything and weight next to nothing, and then you know it is time.
We have a 21 year old Himalayan in early renal failure according to bloodwork. She is 4.5 lbs and has no muscle mass whatsoever. She is incontinent and has trouble supporting her rear end. We told her breeder how old she is and she outlived all of her siblings and lived longer than her parents did. I think it is time, but my mom and sister are having a hard time with that. Siiiigh.
Drinking and peeing more are the first signs but they are easy to miss and cats are so good at hiding illness that regular blood tests are the most reliable. I have my older cats checked every 6 months or sooner if there’s a problem.
I had a cat with renal failure that I had no idea had it, I was young at the time. The thing that stuck out for me was that my cats box was absolutely insanely full. It was as if the cat was a water producing machine. Also, he didn't really want to be held anymore or play and sat a lot in the muffed up position. Of course he slept all the time as well. Mine didn't really lose that much weight but he was small from the start.
It’s not always an old cat disease. There may be congenital anomalies or the kidneys may be scarred from an infection. You can’t foresee everything, though, all you can do is your best.
Alice is in early CKD, still controlled with just prescription food. The first I knew was blood test results. Alice has always drank more water then most cats so that wasn't reliable. I can now tell from the litter box. She pees a lot more then she used to so I am scooping twice a day now rather then once. I also switched litter to one that clumps better which makes scooping easier.