You should be able to feel his ribs, by running your fingers over his sides. You should be able to see a rib or two.
If you can't feel any ribs with your hand or fingers, sliding your fingers from the front toward the back of the dog, then he is overweight.
Take him for walks everyday, about 45 mintues each time. And, feed him dry dog food, using the amount on the side of the package.
You can determine his weight by either taking him to a Vet, or you can pick him up, and stand on a scale... then subtract your weight from the total of both you and him.
Also, here is a site with pictures, and explanations. http://www.placervillevet.com/canine...0condition.htm
If you are caring for the dog, for your neighbor, are you going to have it very long?
Weight loss takes awhile. If your neighbor gets the dog back, he/she will have to continue doing what you are, in order for the dog to get back to a good weight.