Hello Kal_B.
If it is in fact just a reed switch then an ohm meter should be able to detect open and closed circuit as you rotate the paddle. I work on rainfall sensors that use a reed switch and a tipping bucket with a magnet. The sensor is connected to a datalogger that just counts pulses and the k factor determines how much rainfall per pulse.
Sounds similar to this flow sensor. Do you see any magnets?
The data sheet shows a square wave output which could be accomplished with a reed switch or possible hall sensor as well. Try an ohm meter first and if no change in continuity is detected then try powering it per the wiring diagram and check for square wave output.
rfranzk