
Despite the older dog having a thick coat, your fingers should be able to identify the individual ribs - they shouldn't be like a washboard, but you shouldn't have to prod through wodges of flesh to feel them! It's very hard to describe the correct feeling!
And despite his thick coat, his body should have some shape and definition between his ribs and his hips - when you stroke him from shoulder to hip when he's standing your hand should go over the spring of his ribs (feeling the bones of the ribcage), then do the slight 'eggtimer' shape from the end of the ribs, into the waist, then out again to the hip.
If the top of his back is flat like a tabletop, then he is probably overweight. You spine shouldn't stick out, you shouldn't be able to see the individual vertabrae, but his body should fall gently away from the middle.
Not very well put, I'm afraid! I hope you can understand what I'm trying to say!
:)