Check the ribs. If it's hard to feel underneath the skin, they might be overweight. If it's too evident, they might be underweight. I also weigh my dogs every month, just in case.
Just my 2 cents, I changed to Orijen just 4 days ago, and so far, no loose stools, no watery stools, all good. Even my shihtzu eats it, which is amazing cos she's the pickiest eater of all. I also notice their pee/poo does not smell strongly anymore, if that is significant. I used to feed them Science Diet
I had great advice from Marci about foods
so big thanks to her.