When you turn on the engine the cable will pull the lever and hold it there till 5000ish RPM when it releases it. So to test its operation watch it whilst some one turns the engine on or rev it up too past 5000, but make sure the engine is warm.
Lean down and operate the lever with your fingers you will hear the flap moving inside the inlet manifold. If the lever moves and flaps dont the problem is with your inlet manifold. £600 for a new one of them