You should be able to feel your dogs ribs, sternum, backbone and pelvis as wel as see a slight hourglass shape(smooth down the fur). You have to check all these areas because different dogs carry weight in different areas. I have seen dogs who I can see ribs when they turn who are very overweight because they are carying all they weight in their chest or back.
I explain it like this. Hold your hand out flat, palm down and feel your knuckles that is what the bones on your dog should feel like. Now ball your hand up and feel your knuckles that is a dog too thin. Now open your hand back up, flip it over and feel your palm right right below your fingers that is an overweight dog.
Better to keep growing puppies slightly on the lean side.