Cats hide illness so well that weight is basically your only early warning

A dog that feels bad acts like it. A cat that feels bad goes and sits somewhere, and looks exactly like a cat sitting somewhere. That instinct is why so much feline disease gets caught late. Chronic kidney disease, hyperthyroidism, diabetes and dental pain all develop over months and the cat compensates the entire time. By the point behavior changes in a way you can't miss, it's usually well along. Weight is what moves first and it's the one thing you can actually measure at home. Half a pound off a 10 pound cat is 5 percent of body weight, the equivalent of a 9 pound loss on a 180 pound person, and you will not see it under fur. So weigh them monthly. Stand on a bathroom scale, note it, pick up the cat, stand on it again, subtract. Write it down somewhere you'll keep. That's the whole practice and it'll find things a year before you'd have noticed them any other way. Worth watching alongside it: drinking noticeably more, or bigger clumps in the litter box, which points at kidneys, diabetes or thyroid. Eating well but losing weight anyway, which is the classic hyperthyroid picture in an older cat. Hiding more than usual. Stopping grooming, or overgrooming one particular spot. And no longer jumping onto things they used to jump onto, because arthritis is massively underdiagnosed in cats and cats basically never limp. On water, since it connects. Cats came from desert animals and have a weak thirst drive, so they run mildly dehydrated on dry food, which isn't doing kidneys or bladders any favors over a lifetime. Wet food moves the needle more than any fountain does, though plenty of cats do drink more from moving water. Put the bowl away from the food and use something wide and shallow so their whiskers aren't hitting the sides. Senior cats, meaning around 10 and up, want bloodwork annually even when they look perfect. That's how kidney disease gets caught while there's still something useful to do about it.