How to identifying which magnetometer was being used

Hey all, I am a new user of px4. i have been using ArduCopter until now and I was wondering how I can identify which magnetometer was being used during a flight, by looking at the log.

I have noticed that the sensor id stored in sensor_mag and sensor_mag_1 do not have any significance in the sense that sensor_mag is not necessarily the one being used as main.

I did a test where sensor_mag (external) was unplugged and then plugged it in. It started logging but while it was missing sensor_mag_1 must have been main.