So, if I understand Ascend Charlie and FH1100 Pilot correctly, the splitting needles approach used in pistons, doesn't work for turbines (or at least doesn't give a reliable indication) due to the slower RPM decay of a turbine engine.
The way to check the freewheel unit on a turbine is to rotate the blades backwards (engine off - obviously!) which should rotate the engine, and then forward which shouldn't rotate the engine. This approach can't be used in pistons because they have a clutch which would have disengaged the engine at this point.
It'll be a while before I'll need to know this in practice but it is good to know.
thanks,
Matthew