Without a refractometer, one can tell a potato, or any other food is bad when it will decay readily. How prone to decay is INVERSELY CORRELATED with quality.
Check my recent posts regarding gleaning and dead-fall fruit, also my post around 2 yrs ago regarding the longevity of hi brix fuzzy patty pan squash stored without refrigeration.
Keep seeking, applying, sharing accurate knowledge in order to
keep seeking abundant life and joy!