Get information when the primary energy source fails

I am programming a script that needs to be aware if the stream is using the primary or secondary energy source (Mode 3). I have tried using the serialless-mode and read from the GPIO but so far I fail to properly connect/program it as I am always reading 1 in the GPIO output.
I was wondering if I can use, instead, the power fail warning. How could I read it in my script?

Hi,
sorry for my delayed answer - as for this function you can use the powerfail warning (so you check if it triggers in the powerpath transition and saves the state in a variable in the script), but more reliable would be a method to read out the actual active powerpath in the StromPi. We'll release a new firmware in the next few days, where this function would be include.
Best Regards,
Jarek