How can hang gliders and paraglider pilots prevent stall?
Paraglider pilots learn "active flying", whereby you use feedback from the wing to determine what's going on. You monitor the tension in both controls (the "brakes"), and the wing's attitude (if the wing pitches forwards then you feel like you are swinging backwards).

In bumpy air (such as on the edges of thermals, or in wave) active flying is used to keep the wing more or less above your head. If the wing surges forwards then you risk a collapse; if the wing falls back then you risk a stall.

In smooth air a stall is only likely if you are pulling the brakes down too far. This can happen accidentally, especially in nil-wind landings if you mistake your ground speed for airspeed.