What you will see and feel if your dog is too thin:

Ribs:You can see bones protruding and don’t feel a small amount of muscle/fat over them when you run your hands along your dog’s sides.
Base of Tail: When looking at the base of the tail you see bones sticking out and don’t feel a small layer of fat/muscle covering the base when you run your hands along it.
Spine: When looking from above you see bones protruding and don’t feel a little coating of fat/muscle covering the spine when you run your hands along your dog’s back.
Waist: When looking at dog the from above you notice an extreme waist line and bones are visible.
Abdominal Tuck: When looking at your dog from the side you find a serious tuck under the ribs.
What you will see and feel if your dog reaches a healthy weight:

Ribs: When running your hands along your dog’s side while setting a small number of fat you can easily feel the ribs.

Base of Tail:When setting the base of your dog’s tail you notice a small layer of fat and it is smooth to the touch.
Spine:When running your hands along your dog’s back you should be able to feel the spine and a tiny layer of muscle/fat covering it.
Waist: When looking at your dog from above you see a visible waist line just behind the ribs.
Abdominal Tuck: When looking at your dog from the side the area behind the ribs is visibly smaller than the torso.
What you will see and feel if your dog is obese :

Ribs: When running your hands along your dog’s ribs you can’t easily feel the ribs or feel them at all.
Base of Tail: When setting the base of the tail there’s a large amount of fat covering it and no bones are felt.
Spine:When running your hands along your dog’s back no bones can easily be felt below a layer of fat.
Waist: When looking at your dog from over no waist is seen or if the area between the ribs and hips protrudes.
Abdominal Tuck:When looking at your dog from the side There’s No visible tuck behind the ribs or if there is a layer of low hanging fat