First off - don't look at their bellies. Healthy goats *should* have a wedge shape about them - with their rumens (stomaches) being the widest part. As goats eat they swallow a lot whole and in long pieces, which stretches out their stomaches to be very wide at times. As the hours after eating go on, goats bring their food back up and chew it again (cud), breaking into smaller pieces and more compact, causing their stomaches to appear now much smaller. In this way it may appear that your goat is fat and overweight just a few hours after receiving their daily hay ration. And for the same goat just hours before feeding may appear skinny, or in good weight. So - Don't look at their stomaches to tell you if they are over weight, in truth goats carry very little fat on their mid section.
Fat is generally carried in the shoulder and neck area. However, this area can be a tricky to judge on unless you know your goats and their lineage. As many dairy breeds, especially from heavier milking lines, will naturally have a lot of loose and large wrinkles in their skin at the neck and chest area. You may try and see if you can grab a large fistful of flesh between the front legs and lower chest area to help tell.
I however find the best and easiest way to tell is by the backbone, just before the hips. Run your hand along the top of the back, back and forth right before the hips on your goat. You should always be able to find and feel the back bone. If you can't feel the bone, or it takes some digging to find, you goat could stand to loose some weight. If however, the bones stand out and may even feel sharp to touch - your goat is underweight. The bones should be easy to find/feel, but yet have a nice layer of skin over them and not feel sharp for a goat with a healthy weight.
