1. Using the bathroom outside the litter box
Cats that urinate outside the litter box are typically either stressed or sick. A marked increase or decrease in urine or stool, the presence of blood or mucous, or a particularly pungent smell (when the box has been recently cleaned) are all warnings of possible trouble ahead.
2. Dropping food while eating
If your cat has been steadily eating the same diet, then suddenly loses enthusiasm, don't assume she's just gotten tired of the same old food. Going off her food is another way your cat lets you know she's not feeling well.
3. Losing weight
Irregularities in weight gain point to kitty discontent or sickness. There are many things that can cause weight loss, including the cat's quality of food, and/or chronic disease.
4. Decreasing urination
If you notice that your kitty is making frequent trips to the litter box with little to show for it, it could be a blocked urinary tract or dehydration, and should be taken to the vet.
5. Ceasing to groom
Fastidious groomers letting themselves go is a sure sign of potential illness. You can usually spot the signs quickly, as a once-silky coat will feel harsh or greasy.
If you notice any of these symptoms, then a visit with your veterinarian should be your next step. Any one of these signs could mean a serious medical problem.