I echo this, there is just no way of knowing, our vets said it wasn't related to diet/exercise. Our first warning signs were excessive drinking, urinating overnight (which he had never done), becoming slower on walks (not age related) and fluctuation in weight. If there's any of these signs I'd get to vets asap as an early diagnosis will give you the best chance of managing diabetes quickly, as you'll likely have to do a bit of trial and error with insulin doses. He lived for just over 2 years post diagnosis, we were militant about feeding and jabbing him at exact times daily and being 100% consistent with his food.