If you believe your cat is overweight, run your hand along his flank.   You will want to be able to feel the ribs, but not see them.  You can also look at him from the side.  The waist should look narrow.  And if you look at the cat from the top, there should also be a narrowing of the waist.   If you do all three of these, but you do not feel the ribs or see a narrowing of the waist, there is a good chance that your cat is overweight.  You can also check with your vet to see if he could stand to lose a few pounds.