It is really enlightening to see the varied opinions from so many people. Let me put in my 2 cents worth. Before I get drilled from the other highly regarded participants, I am going to give what I know to be a likely possibility from years of maintenance experience as well as flying experience in the type.
This applies to a Bell 206 and the RR 250 engiine.
First: when the twist grip is rolled to flight idle the governor is out of the system.
Second: Once the twist grip is in the flight idle positionwhile in flight (autorotation), the N2 RPM will be TOTALLY dependant on what maintenance has the Fuel Control (N1) IDLE setting at. If the FCU idle is on the high side, there will be NO needle split in autorotation. For training ships, it is generally a good idea to set idle on the low side to get a good needle split in autorotation.
Third: Don't ever fly a machine if you suspect the freewheeling unit to be defective. The life you save may be your OWN! You can check it on the ground as others have stated by rotating the rotor both directions and listening/observing the 4th stage turbine wheel in the exhaust stack. Rotate the rotor IN the direction of rotation and the turbine should NOT rotate, and there should not be any rubbing/chattering noises. Rotate the rotor opposite the direction of rotoation, and the turbine SHOULD turn, and you MAY get some additional "whirring"noise from the engine.
Most importantly what I have drawn from this thread is if you have a maintenance question, ASK A MAINTENANCE TYPE!!!! Pilots are notorius for trying to baffle with BS when they aren't sure. Yes, I am an ATP. :-)