All normal chickens are "skinny" compared to a Cornish Cross.
Here's a link to one body condition chart:
http://www.poultrydvm.com/featured-infographic/chicken-body-condition-score-chart
If the keel bones are sticking out way beyond the flesh of their breasts, that could indicate skinny. But if you just mean you can feel that there is a keel bone, that's normal for healthy layers. I think it's a bit like feeling the ribs on a dog or person-- they are there, and you should be able to feel them, but they shouldn't be sticking way out.
Chickens that are active and seem healthy often are healthy.
Chickens that are laying eggs often are healthy.
They might really have been getting enough food when free-ranging and eating scraps, so they didn't "need" to eat what you were providing. Summer is the time when chickens can most easily find food while free ranging. I would expect them to need more purchased feed in the winter than the summer, but I would also expect that need to gradually increase during the fall months, not appear all at once. So they might (maybe) gradually increase their consumption of purchased feed as the food available from ranging goes down.
The only way I know to check if a chicken is getting the right balance of nutrients to be healthy is to look at her appearance, behavior, and whether she's laying, along with feeling for body condition. (If the chicken dies for some reason, you can check the level of fat inside her abdomen to get some idea of whether she's been getting enough total calories. But that still doesn't tell if the balance of nutrients was correct or not, so not terribly useful.)
