How to analyze which GPS source (in my case i have 2 GPS onboard) was used by EKF3?

ArduCopter
,
1

I’m working on the smooth switching between GPS1 and GPS2, and I need to be sure that ONLY 1 GPS source is used by EKF3.

What I’ve done:
I make an in-flight change GPS_PRIMARY from 0 to 1 and see that the drone is flying, but I am not sure that EKF predictions are based ONLY on the second GPS.

Questions:
1. How can I check it or track it?
2. Can I control which GPS exactly to use for EKF3?