the pancake fan has spiders (legs) that allow you to secure a sensor.
use some whitehout on a fan near the hub, or paint a slice of pizza on the hub (triangle)
a reflective sensor is common on machines such as lathes for this RPM feedback.
of course it does not provide airflow, only RPM.
there are lots of information on thermal anemometers using simple (cheap) diodes.
easy to determine that there is flow. you do not need a 3rd heating element as you know the direction already, all you want to know of there is flow. google kings law