Quick answer: You can check by feeling your cat's ribs — if you can't feel them easily under a thin layer of fat, your cat is likely overweight.
Run your fingers along your cat's sides. You should feel the ribs with just a thin fat layer. If you can't feel them, or if your cat has a visible belly pouch and no visible waist from above, it's likely overweight. A healthy cat should have a slight tuck at the abdomen.
Indoor cats that are spayed or neutered have lower energy needs than outdoor cats. Overfeeding, free-feeding dry food, and too many treats are the main causes. Some cats also gain weight due to age or hormonal changes.
Switch to measured meals twice a day and reduce treats to less than 10% of daily calories. Add at least two 10-minute play sessions per day. Consult your vet for a weight target — rapid weight loss in cats can cause serious liver problems, so changes should be gradual.
Most overweight cats reach a healthy weight within 6 to 12 months with portion control and more activity. A vet visit is the safest starting point.