Cats can have a variety of health problems, some of which can defy detection.
Cats do not reveal they are in pain, unless it is extreme, so it is up to us to be vigilant in our observations.
Use your 5 senses to notice problems:
1. Smell - If your kitty has a bad odor, take note of its source. It could be bad breath... signaling a dental or oral problem, or a digestive problem. Body odor can mean the cat is not self-grooming, perhaps due to arthritis or obesity.
2. Sight - Watch for changes in weight, especially drastic weight loss, for signs of underlying health problems. Notice eating habits, elimination behaviors, changes in sleep patterns and overall energy levels.
3. Listen to your cat's breathing. If there are sounds of congestion in the lungs, or a stuffy nose, it could mean a respiratory illness. Also be mindful of your cat trying to get your attention by whining or whimpering. Sometimes they "ask for help" by vocalizing.
4. Touch - Regularly pet your cat and notice if the fur is as smooth as usual. If the coat is oily, or dry, or if there is significant dandruff, there might be a skin problem. Also feel for lumps under the skin, sore spots (kitty will let you know if you touch a painful place), and for general condition. Sometimes you will notice weight loss by feel before you see it, especially if kitty has long hair.
5. Taste - Well, let's not go there. However, you might take a moment to see if kitty is still interested in the usual foods. Cats can lose interest in food if it doesn't taste right, and that can signal illness, too.
In all cases, if you notice any problems in your observations, take your cat to the veterinarian as soon as possible. When cats get sick, they can deteriorate rather quickly. Don't wait until it's too late.