Hi James!
I would advise discussing a weight management plan with you Vet, there are special foods and dietary plans available to help manage dog weight. As they get older it becomes a lot easier to gain weight and can easily creep on. I have attached a general body conditions coring diagram to give you an idea of what to look for but as with most cases, each is individual and you have to take into account factors such as fur thickness, normal body weight, activity.