A cat can't tell you in words when it doesn't feel well. Your cat doesn't have to talk to tell you that it is sick. If you know what to look for, your cat can tell you a lot about how it is feeling.
A lot of vets get calls about cats that are not acting normally. This doesn't tell the vet anything. A cat may act one way at home. Then it gets to the vet's office, and nothing seems to be wrong. The better you can describe what is going on, the more the vet can help you. Write down exactly how it is acting. Be as clear as you can be.
Pay attention to how much your cat eats. Also notice how it acts when it goes to the food dish. Is the cat not interested in food? Or does the cat come running at food time but then doesn't eat anything? In the first case, the cat has no appetite. In the second case, it seems interested in food. It just doesn't eat it. They sound the same but are really two different problems.
A cat that hasn't been eating well might not be drinking enough water either. You can check to see if your cat is drinking enough very easily. Gently grab the skin between the cat's shoulder blades. Pull up a little bit. If the skin goes right back into place, the cat is drinking enough. If the skin does not go back into place, it probably isn't drinking enough water. It would be a good idea to call your vet.
A healthy cat has a soft, clean coat. A cat whose fur is dirty, dry, or dull may not be getting groomed enough. The cat could also be feeling sick. Cats usually work hard to keep themselves clean. They usually only need a little brushing to help. A cat that isn't keeping itself clean is not lazy. It probably doesn't feel well.