How to tell bird is thin?

So, to me, near all my birds seem thin. How do I exactly tell my birds are thin or not? Seems like the bone in the middle sticks out on all of them. How much should I feel on their breasts? I just don't know...
If I had a 5lbs and smaller scale, like a kitchen scale, I would use that and known weights. But I don't have one, so how else can I know?
I feed layer crumbles from Tractor Supply, 2 cups of scratch a day. There are about 30 birds total, most are under 9weeks. They don't free range, but have a 20 by 40ft run. I give them treats 2-4x a week, yogurt, bread, old food, etc.

 

There is a feeder for the chicks with chick food, they rarely go eat the adult food.
For 30 chickens, 2cups doesn't seem like a lot. Maybe that's just me. I read somewhere that someone was giving 1.5 cups and they had 10-15 birds.

There is nothing on the birds, all look clean, nothing at the skin anywhere on them.

 

They are bantams, so the human scale will be more off. It is already 5lbs off, more or less for a person. We have to weight ourselves a few times and clear the scale before each time, then pick the median, lol. I don't think it will really notice Japs and tiny babies...unless we held a few..
When my dog killed 10 of my birds, I cut one open, not really worth the bother, even to feed to the dogs...

 

Bought chick starter/grower. Bag says after 6weeks, I should be feeding...er...idk, but not starter and not layer...ugh!