How do I identify which neutral wire to attach to my GFCI breaker?
First and most importantly:
SHUT OFF THE MAIN BREAKER IN THE PANEL BEFORE YOU START POKING AROUND IN IT!
Then you should trace the HOT wire from the breaker back to where it enters the panel.
Once you have found that you should be able to identify its NEUTRAL and GROUND wires.
Trace those back to where they connect to a bus bar.