You want to feel them. If their ribs feel sharp, then they are under weight. If their ribs feel more like pencils in a pencil case, that is good. If there's a thick pad of fat and it's hard to feel the ribs it is too fat. If they are fat they will also have trouble keeping the under side well cleaned.
There are some breeds that have large dewlaps and/or a little skirt around the bum, so that is not a good marker of being over weight. Sophie was just declared a great weight by the vet and she has both because of her mix of breeds.
Rabbits are great fakers when it comes to food. You would swear mine are constantly starving if you went by behavior at meal times. I know they are not because they are all good weights, but more importantly, they ALWAYS have lots of hay. If they were that hungry they would eat hay rather than starve. You want them to consistantly munch the hay rather than scarfing pellets all day.
