poor skin turgor (when the skin on the back of the hand stays elevated after it is pulled up and released) (wehavekids.com)
Signs of severe dehydration ie: lack of skin turgor (skin doesn't settle back down quickly when the back of hand or neck is pinched) or dryness of the gums, mouth, and tongue requires immediate emergency intervention. (kitchentablemedicine.com)
While more serious levels of dehydration will show reduced skin turgor or «tenting of the skin» when pinched. (dralisonchen.com)