What bluetooth audio codec does iOS use for Airpods? I recently bought the ToothFairy App and noticed that it has an option to enforce the AAC codec. The difference in sound quality is remarkable. 

How can I tell which is being used between iOS and AirPods?
You can't tell which codec is used on iOS, but AirPods use SBC or AAC. 
On macOS, you can check which one is in use by going to the Bluetooth menu, clicking on your Airpods with the "Alt" key pressed.