Unlike birds we humans don't have an instinct for measuring heights in free air. That's why for instance paraglider- and hanggliderpilots use a so called variometer. With this device you can get a confirmation if your flying in a weak thermal or that you're sinking less faster then a few moments before.
Most important, of course, is that you try to look as less as possible to that little green screen and try to enjoy as much as you can your free flight. But that goes without saying.