Stress manifests when the productivity of an individual is below the belt or not up to satisfaction. It is usually measure on performance, either job or academic related.
There exist clear psychophysical measures for stress; especially mental stress is the physical provocateur of cancer and other 'civilizational' mass (epidemic) diseases. My favorite experimental researcher is H.Selye as he was among the scientific pioneers, but psycho-medical stress research today is a great field, e.g. salutogenesis is a key concept for the future.