I'd say that a bird is physically fit when able to survive predator attacks for many years while free-ranging.I want to try not to conflate overall health with physical fitness.
We humans, at those of us old enough to see cardiologist and other doctors, are given physical fitness tests and told to exercise to a cerrtain elevated heart rate for twenty to thirty minutes multiple times per week.. There are defined measures of physical fitness based on age and timed running and such. You can find them on the internet.
How would we know a bird is physically fit other than by the looks of it?
Or did you have some kind of agility/fitness-parcours for chickens in mind?