Most dogs won't over-eat unless you give them something so 'tasty' that their tastebuds OVERRIDE their "Leptin" levels.
Leptin drives down the appetiite in the dog, once it's hit a lean body mass.
It keeps the dogs appetite in check so it doesn't get too fat to run. Run after rabbits, run away from bears.
But you can defeat the Leptin impact with bacon, vanilla icecream, Skittles candies, and lard. And wet food mixed in the dry.
SO Dr Johnson gives you symptoms of overweight to look for. Actually ONE symptom and that's based on the body shape. Says dogs should have a waist and specifies a 'good' dimension.
Here's the article on "Why Dogs Get Fat, and How They're Supposed to Eat" at Drjohnson.com