Use validated vocal metrics used by speech pathologists and ENTs to determine vocal health. Track your vocal range, vocal smoothness, and vocal stamina to see changes over time.
Take quizzes used by vocal health professionals to find out how your voice changes day-to-day. Compare results with your lifestyle data to find out whether certain habits might be having an effect on your vocal health.
Track variables to check how your vocal quality is uniquely affected by certain factors. Does getting enough sleep make a big difference? How about how long you warm up for? Stretching in the morning? Hydration? It all depends on you and your voice.