This is what my vet told me:
(1) Make a fist and feel your knuckles...If their back/ribs feel like this = under weight
(2) Put your hand straight out & palm down and feel your knuckles...If their back/ribs feel like this = Just right
(3) Put your hand straight out & palm up and feel your "knuckles"...If their back/ribs feel like this = over weight
Just advice
our vet gave us on how to tell