Signs of dehydration include dry mouth, thirst, decreased urinary output, dry skin, constipation, headache, lightheadedness, dizziness, tiredness and sleepiness, according to Mayo Clinic. In children, no wet diapers for several hours, few tears when crying and decreased activity are indicators of dehydration.
The Mayo Clinic cautions that dehydration can be a medical emergency in its most severe state. Signs like very dry mucous membranes, skin and mouth, extreme thirst, dark urine, little or no urine output, sunken eyes, skin with no elasticity, rapid heartbeat and breathing, fever, delirium and unconsciousness can be signs of a serious problem. In infants, sunken fontanels, which are sometimes called "soft spots" can be indicative of dehydration as can producing no tears when crying. Infants or children are generally extremely sleepy or fussy when dehydrated.
A healthy adult can generally treat mild or even moderate hydration at home by drinking additional fluids, including water and electrolyte drinks. Still, signs of severe dehydration shouldn't be ignored and require evaluation by a medical professional.
Older adults and children should be seen right away if they develop a bloody or black stool, can't keep fluids down, develop diarrhea that lasts for longer than 24 hours or seem disoriented.