Signs of dehydration during exercise include increased thirst, dry mouth, fatigue, and decreased urine output, often indicated by darker urine color. Athletes may experience muscle cramps, dizziness, or confusion as dehydration progresses. Monitoring these signs is crucial, as even mild dehydration can impair performance and cognitive function. It's important to hydrate regularly during physical activity, ensuring fluid intake matches the intensity and duration of the exercise to maintain optimal performance level and overall health.