Obesity is the leading cause of sickness in the United States, not just for humans but for our pets as well. Keeping a dog healthy is just as important as keeping ourselves healthy. However, determining the health of a pet can be much more difficult than assessing the health of a person. As humans, we are able to talk and communicate about what's wrong and how we feel. Dogs lack the ability to communicate their problems, so it's up to their owners to determine how healthy they are. There are several steps that can help determine whether a dog is overweight.
Look at the dog's ribs. When you feel for the ribs, you should be able to feel each individual rib, with a thin layer of fat over the bone. If you can see each rib, the dog is too thin; if you cannot feel any ribs at all, the dog is too heavy.
Check the dog's back near the base of the tail. There will be a little fat covering the area. This is a sign of a healthy dog. Again, if you can actually see the bone sticking out, the dog is too thin; if you are unable to locate any bones while feeling the area, the dog is too heavy.
Feel around the dog's spine, hips and shoulders. Like the ribs, these should be covered in a thin layer of fat, but you should still be able to locate them with your hand. If these bones stick out and are not covered in that thin layer of fat, the dog is too thin. If those bones are hidden from view and you cannot locate them by feeling around, the dog is overweight.
Look at the dog from above. There should be a noticeable taper at the base of the ribs that widens at the hips. The dog should have an "hourglass figure," so to speak. This is a healthy dog. If the ribs are too visible, or if the body doesn't taper between the hips and ribs, the dog is unhealthy.
Look at the dog from the side. Like the taper from above, from the side the dog should have a smaller diameter around the waist than around the ribs. Several types of dogs have a very distinct abdominal tuck; with others you have to look a little more closely.