It's easy to tell the difference between locked and spinning Fit Flight shafts because the spin shafts have a rounded top and the locked shafts are flat on top.
Fit Flight shafts are measured by shaft only length which is the distance between the flight and the barrel. It does not include the threads or the push-in area for the flight.
If you install flights onto a Locked style shaft and then onto a Spin style shaft, you may notice the flight is looser than normal and may pop off easier. You can avoid this by using separate flights for Locked and Spin shafts.
Fit shafts will only work with Fit Flights.