How can I tell if I am dehydrated?
Useful everyday signs include thirst, darker urine, dry mouth, headache, lightheadedness, unusual fatigue, and reduced training performance. Urine color is imperfect, but pale yellow is usually a reasonable sign that hydration is adequate; very dark urine or very low urine output deserves attention.
In strenuous training, heat, elevation, or long outdoor work, body-weight change across the session is a better tool. Losing more than a small percentage of body weight from sweat can impair performance, and repeated under-replacement can accumulate across days.
Severe symptoms, such as confusion, fainting, inability to keep fluids down, heat illness signs, chest pain, or persistent vomiting/diarrhea, are not things to address exclusively with fitness guidance. They warrant medical care.