The first thing to understand is the ideal weight for your dog. The ideal weight however is not a specific number nor is it the same for every dog. Ideally your dogs ribs should be easy to feel, but should not necessarily be seen (this can vary depending on the dog, some dogs are very visible ribs, but this does not necessarily mean they are underweight, this will also depend on the breed), they would also have a relatedly defined waist. There are many charts available and often seen on the wall of your vet which depict the correct shape of your dog.
Your dog should be getting enough exercise for their particular breed. Not only is exercise important for your dog anatomically, but it is also very important for their mental health. Most dogs need far more than a leisurely stroll around the park or neighborhood and some even need more than a run with their owner on foot or by bicycle.
Diet, we control what our dogs eat and therefore it is our responsibility to ensure they are getting the right amount of the right food, make sure that your dog is receiving the correct amount of food, try to avoid giving your dog extras when you are eating or when they are sniffing around the kitchen. It is also best to feed your dog on a schedule and need your dogs food and snacks healthy!
We owners should also remember to monitor our dogs weight, it is important to check your dogs weight regularly to ensure that they have not gained or lost too much weight.