Severe dehydration will be visible by slightly pinching the skin and observing what happens. It should immediately snap back to original form; dehydration will result in it slightly retaining the pinch mark (along with the other signs already mentioned).
Analog hygrometers do just fine as long as you are considering a few factors...hot side hide will be significantly less humid due to the UTH, a hygrometer on the wall could be inaccurate if you have damp substrate, and a hygrometer resting on the substrate will also be inaccurate if the substrate is damp (will look higher than the ambient humidity). Also, google "salt test" for hygrometers; it's a way to calibrate the hygrometer you already have.