24 vdc fan
I remember doing this many years ago. I had a small vane attached to a sensitive microswitch, and placed it in front of the fan, even running slowly it manage to move the vane and operate the switch. Did not obstruct the airflow too much. I suppose you could even use a small magnet on a vane, to operate a reed switch. Only low current though. The above of course depends on whether you have the room to mount such a device. Have you actually instead considered monitoring the temperature of whatever you are trying to cool.
Added after 3 minutes:
There is one problem I forgot to mention, you need to give the fan enough time to run up to speed, before checking any switch.