A cat at a healthy weight has a visible waist when you look down at him, and you can easily feel--but not see--his ribs. Feed a little more if he looks skinny. Feed a little less if he looks like a sausage on toothpicks.
Measure your cat's food and feed him twice a day rather than leaving food out all the time, and take away whatever's left in his bowl if he hasn't finished it in 20 minutes or so. It helps prevent obesity and keeps the food from spoiling.
No single food is right for every cat. Cats are individuals, and even within breeds, each one has unique nutritional needs. Just because your last cat did great on Susie's Supper for Siamese doesn't mean that your next one will. You may have to feed him Sarah's Siamese Special. Be willing to try different foods until you hit on the one that's right for your cat. You'll know by his bright eyes, shiny coat, and high energy.