One solution I've seen, come to think of it, for a PC power supply
fan;

  Put a thin (mylar?) flap outside the case, with an outward-blowing
fan.  Put an opto-interruptor on a frame below it;  If the flap drops,
the opto is interrupted.  Fan spindown then causes a cheapie alarm to go
off.

  (One place just has little ribbons tied to each fan's protective grid,
but that requires an occasional glance to check.)

  Once in a while a fan's blade assembly will split and pop off the
hub.  One Co. told me they lost a old 386 server due to this, the motor
was still spinning (they say - ??) - but the blade assembly had split
and managed to not contact the hub any more (More usually the blades hit
the housing & make this awful racket.)  In a way this is a better
solution; if you power the cheap alarm off the AC output of the power
supply it handles turning it off automatically, and it's a LOT simpler
than watching blade RPM's etc. to just assure that enough air flow is
happening for sufficient cooling of the PC.

  Could also use a magnet & reed switch (magnet on the flap of course
<G>)

  Mark