The tool you need is the Body Condition Score. Body Condition uses the shape of your dog as an indication of where they are depositing fat and if it is in a way that is healthy for your dog.
It’s easy to tell when a dog is underweight. You will be able to easily see and feel their ribs and spine and they may have a sunken look from a lack of muscle mass.
It can be a little more difficult to tell when your dog is creeping into overweight territory. This is partially due to differences in breed and build but is also because most people have no idea of what a dog looks like at their ideal weight. It may be suprisinng to learn that many dogs in the US are overweight or obese. Most often, this is because dog owners don’t have a good understanding of a dog’s ideal weight.
A dog at a healthy weight has ribs that are palpable but not visible. This means you can feel but not see them. You’ll see a visible taper at the waist and an abdomen that is tucked up when viewing from the side.