You can use a DC current sensing switch, anywhere on the Fan feed power wire. Once the switch trips (meaning the fan is actually working) it will complete the circuit to the LED indicator (grd or pwr) whichever you prefer.
There are a few thousand types and sizes of these. A bit harder to find a DC 12/24vdc one though.
This is one:
https://www.amazon.com/Current-Sensi...09YN9Q9YP?th=1