well i think if you have it powered up you have power that you can use
to trip a relay, hook into the switch!.
or are you talking about the spin down ?
there are a couple of ways..
one is to place a hall effect or some optical sensor on the shaft ,
or place a round magnet around the shaft and use the spinning effects
to push onto a steel switch.
the other ways is to make a circuit that monitors the regeneration
of the motor as it spins down.. motors will act like generator when you
disconnect power , while waiting for it to spin down i will generate
AC.
so if you were to make a low voltage monitor device that will also
handle the line voltage when on you have a good indication of when the
blade stops spinning.
using a simple OP-Amp protecting the inputs via high value resisters
into a zener you can drive a voltage comparator..
etc...
if interested i guess you could ask more about it.