Is there a small portable device that can measure hydration levels to see if someone is drinking not enough or too much water?

Unfortunately no: Unfortunately to get an adequate intake and output for hydration status, the best way is to measure accurately what you're taking in, what you are output. Other methods for such a hydration status is obtaining orthostatic tilt test, measuring blood pressure, heart rate between supine, seated, and standing positions, assessing for skin turgor, moisture in the mucous membrane.