This is so subjective, though, and freaks folks out as most layers have an easily felt keel bone, and it's normal. We have had so many folks on here over the years freaking out because they can feel the keel bone and the bird is a perfectly normal weight. If you're really, truly concerned, weigh...
As a rule, most chickens aren't going to overeat. I think corn and scratch grains are the main culprits for overeating, so limit those. But if all they have is commercial feed and some kitchen scraps, they're not really going to overeat.
If your birds are active, alert, glossy feathers...