If they run like a fat lady(lumbering, chest slightly low for balance, moving side to side on their legs), they are too fat...if they run with good scissor action on those legs like a young girl, bodies level to the ground, they are fighting fit. If you watch your chickens enough and note how they look, feel and move when at a normal weight, you can also tell pretty easily what is abnormal. If you have to weigh them to determine what is normal per some book or standard, it's likely you will be doing more work than you need to do all through the years you have chickens.
Their weight changes through the seasons and as they age, so a prominent keel on a young bird is normal. A recessed keel is most likely only to be found only on a meat bird. A slightly palpable, but not overly so, keel on any adult bird is about right. You'll feel the keel more on a layer breed than a dual purpose.
The more you process your own birds, the more you grow familiar with what is fat vs. what is meat, the difference between roosters and hens regarding fat deposits, the keel shape and prominence for specific ages, breeds and sexes.
Chickens can and do overeat depending upon breed and feed availability, so it's prudent to monitor body conditioning but it's something that comes with time and experience. Watch your birds, feel of their bodies, note their mobility, their ease in getting up on and down off the roost, running, breeding, etc. Watch all that and see how you can compare it to humans with similar mobility problems and how they move when they are obese and you'll soon see it applies to chickens also.
