The tail should bulge a little as it leaves the body, if it tapers straight away, he's thin. This is an older picture of one of my sub-adults, and you can see the fat in the base of his tail:
He has actually grown a little longer and put on weight since the pic, and he's even beefier looking now.
The hip bones and spine should not be visible.
He should be well-muscled, as can be seen in the legs of the BD above, and having enough meat on his bones is what makes the fingers/toes look more proportionate.
The head should not look "too big" for the body in anything but very young babies. As they grow, the head begins to look proportionally smaller, so if your adult or sub-adult has a big-looking head, he is probably underweight. Note that in the top pic, his head actually looks a little small.
There are fat reserves on the head, and a healthy lizard will have two symmetrical bulges above and behind his eyes, just ahead of the spiked edge. If the head is flat or slightly concave there, you have a skinny lizard.
Hydration is very important to being healthy and properly metabolizing nutrients. The skin should more or less lie flat to the body, and it there is excessive wrinkling when he lies flat and straight, or if his eyes appear to be sunken in at all, he is very probably dehydrated.
This sub-adult is an adoption I recently took in:
As you can see, he is quite skinny, and he's actually a few months older than the BD in the first pic. Here's what I see that indicates he is not healthy and is underweight:
His head looks big and kind of bony.
His tail is narrow and tapers straight back from his body.
His hip bones are obvious.
His legs are thin.
His fingers/toes look too long.
His body lacks mass.
His skin is wrinkled, and it's not due to his posture.
This lizard is slowly improving, but I suspect his temps weren't quite right and that he didn't eat as well as he should.