Do you know how to test the condition score on a sheep? You push on the spine where it meets the pelvis, just above the tail, and feel for how much skin and fat there is between the surface and the bones. You want it to feel about like a healthy human's forearm, if you were feeling the bones through the thicker part of the muscle, or so I was shown. It's kind of hard to explain but that's the general idea - if it's too poky, like it feels sharp, they need more, if you can't even feel the bones, they are too fat, and if you can feel the bones but they have a nice cushion, that's perfect.
I just recently got a Dexter, and wasn't at all sure how to tell what condition she was in. Our 4H beef leader showed me how to test the same way I do on a sheep, but on the short ribs on the cow's side. If you can feel the tips of those bones as sharp, they need more feed ... if you can't feel 'em at all, they are too fat, and if they are just nicely defined, then you're good.
Perhaps if you look up condition scoring you'll find some better explanations than my poor attempts.