Advice about drinking water is abundant, while the question of how anyone would know their hydration status gets much less attention. The measures in common use each capture a different piece of the picture.
The body regulates concentration, not volume
Fluid balance is defended primarily by keeping the concentration of dissolved particles in the blood within a narrow range. Sensors in the brain track that concentration continuously.
When concentration rises, the brain triggers thirst and signals the kidneys to retain water by producing more concentrated urine. Both responses restore concentration rather than any target volume.
This is why total body water can vary considerably between people of the same size without any of them being dehydrated. The regulated variable is concentration.
Thirst is a real signal with a lag
Thirst appears when blood concentration rises past a threshold, which is a genuine physiological signal rather than a folk belief. It is not, however, an early warning system.
The sensation typically becomes noticeable after some water has already been lost, and it can be blunted in older adults, whose thirst response tends to be less sensitive.
Thirst also switches off before the body has fully replaced what it lost, because drinking itself sends signals from the mouth and stomach that dampen the sensation.
Urine color measures the kidney's response
Urine darkens as the kidneys conserve water, so color is an indirect readout of how hard that conservation is working rather than a direct measure of body water.
The reading is easily confounded. Some vitamins, medications and foods change urine color regardless of hydration, and a large drink produces pale urine within an hour of consumption.
Laboratories use urine concentration measured by instrument instead, which removes the guesswork but requires a sample and equipment rather than a glance.
Short-term weight change tracks water most closely
Over hours, body mass changes mainly through fluid. Weighing before and after a long workout gives a reasonable estimate of sweat losses that were not replaced.
Athletic and occupational settings use this method because it is cheap and sensitive over short intervals. Its accuracy degrades across days, when food and waste also shift weight.
Repeated morning weights taken under consistent conditions are the version most often used to spot a running fluid deficit.
Where the question needs a clinician
Dehydration that produces dizziness, confusion, very low urine output or persistent vomiting is a medical situation, not a self-monitoring exercise, and warrants prompt professional attention.
Kidney disease, heart failure and certain medications change how fluid should be managed, sometimes in the opposite direction from general advice. Those situations require individualized guidance.
For everyone else, the practical value of understanding these measures is knowing what each one does and does not establish.