While monitoring your actual output isn’t the most practical thing, watching the color of your urine output is often the best indicator of hydration status. See attached chart. Note that excess vitamins and minerals can alter the color of urine output.

Drinking to thirst is a common way to stay hydrated. Thirst is triggered by fluid shifts and electrolyte imbalances. When thirst is felt, the average person is already dehydrated by ~2%. While this not an issue for the average person, for an athlete, it can cause performance issues. Especially if the dehydration was caused by fluid loss due to exercise. In studies comparing humans & animals drinking ad libitum, humans tend to under-replace their fluid needs over the short term. Thus, fluid intake evenly spaced throughout the day with a larger emphasis around and training, training may improve athletic performance. However, drinking to thirst is fine for the general population.

Further Reading
  1. https://www.nap.edu/read/10925/chapter/1
References
  1. http://www.cism-milsport.org/eng/004_SPORT_AND_SCIENCE/articles-and-pdfs/004-hydration.pdf