Usually clinical
Significantly dehydrated if has 2 or more of the following (LR+ 6.1, CI:3.8-9.8):
Prolonged capillary refill (>2 sec)
Dry mucous membranes
Absence of tears
Abnormal overall appearance
Sunken fontanelle in an infant
Acute gastroenteritis 
Esophageal stenosis/atresia
Bacterial gastroenteritis
Pyloric stenosis
Viral gastroenteritis
Intestinal stenosis/atresia
Acute diarrhea
Duodenal atresia