How do induction cookers measure temperature?
At least some of them appear to have a silicon thermistor set in the center of the induction coil and (probably mostly radiatively **no, see below) thermally coupled to the pot. 

Here is a photo of a replacement Chinese-made one from this site: 



**Edit: The sensor is slathered with heatsink compound and in thermal contact with the bottom of the glass. Screen capture from this video. 



From another video, a different (IKEA 2kW) cook top: 



Whilst an IR sensor might be better under controlled conditions (though that's dubious in general given the very low emissivity of many pot bottoms) it's probably too expensive to use on typical induction cook tops.