You can start determining if your cat's overweight by looking at him. Examining your cat from above and from the side should help you determine if his weight is a problem.  Look at your cat from above. The area between his ribs and hips should move inward slightly, creating a clearly defined waist. If there is no waist or if his waist is wider than either his hips or ribs, your cat may be overweight.  You should also look at your cat from the side. Cats who are at a healthy weight have what is called an abdominal tuck. This means the area just behind their ribs should have a smaller diameter than the chest. If you do not see an abdominal tuck, your cat may be overweight. You can further examine your cat by touching him. Run your hand over your cat's sides. You should be able to easily feel his ribs. If you cannot feel his ribs, or have to push down to do so, your cat may be overweight. You should also be able to feel bones near the base of your cat's tail. While there should be a small layer of fat protecting this area, you should be able to feel the outlines of his bones without much effort. If you cannot easily feel bones here, your cat may be overweight. A cat's spine, hips, and shoulders should also be fairly bony. While very visible and pointy bones may indicate your cat is underweight, in a healthy cat you should be able to easily detect hip bones, spine bones, and shoulder bones through gentle petting. If you're unable to feel these bones due to layers of fat, your cat is overweight. Many cats have a saggy bit of flesh that hangs down between their back legs. If your cat is otherwise bony, this small pouch of skin is unlikely to be an indication of a weight problem. This skin flap is referred to as the "primordial pouch" and its purpose is to protect the stomach during fights with other cats. Cats often kick with their back legs when angry, and this flap of skin protects your cat from such attacks. Many cats will develop a primordial pouch at some point in their lives and it is not necessarily an indicator of feline obesity. However, excess fat may be stored in your cat's primordial pouch if he is overweight. If he shows other symptoms of obesity, check his pouch. It should hang loose and be mostly comprised of skin. If the pouch seems to be filling out with fat, this can be an indicator of obesity.
Look at your cat from different angles. Touch your cat's ribs. Examine the base of your cat's tail. Check other bony areas. Do not be overly concerned about a saggy belly.