You can check the volume status of elderly individual with skin turgor. The best site to check skin turgor on an elderly person is the skin on the clavicle or "collar bone".
Checking skin turgor is done by slightly pinching and raising the skin and observing how quickly the skin returns. The slower it returns, the more dehydrated the patient is. And even better way, check urine
output and color.
Answer assisted by Mazin Albert, Medical Student