I tested my indoor sensor (Davis Airlink) by comparing to others in my neighborhood -- I set it up outside on the first bad air quality day we had and an hour later it was with 5 AQI points of the 4 PurpleAir sensors within a quarter mile.

Certainly not professional level calibration, but shows that it's within the ballpark. Unfortunately I don't think there's an official EPA monitoring station within 10 miles so I can't really check it against a calibrated monitor.