Optical detection of a fan wheel
Omron and others make reflective sensors, an IR emitter and photo-transistor fixed in position in a plastic housing.  They are angled toward each other to set up a detection range.  Some have digital outputs, which might work against you.  One with an analog output, or just the phototransistor pins, could be processed to extract the sinewave signal of a passing fan blade.  That signal drives a comparator that gives you a square wave signal with one pulse per fan blade (not per revolution).
If there is too much interference from the environment, the next step up in complexity is to drive the emitter with a high frequency square wave, something like 100 kHz.  The received signal then is bandpass filtered to reject ambient junk, and peak detected to extract blade information.