I think if you read the B206 RFM it will be there. All checks done at full noise, roll back to idle, check the time for decel, should be around 4-5 sec. If too slow, the freewheel hasn't disengaged, and the rotor is holding it up there.
In pistons, the split is far more observable, the engine drops from 2300 to 800 RPM in a second, while the rotor, with some inertia, holds its revs for longer. But a turbine takes 4-5 sec to go from 58,000 to 40,000 RPM.