Best way to locate a specific neutral?
You should disconnect all power to your sub-panel (turn off your main breaker and your generator). Be absolutely sure the power is off to avoid serious injury or worse. I'd suggest using a voltage sensor pen to verify the panel is cold.

You will need a ohm/continuity meter for this test.

Pop the breaker in question out of the panel, leaving the hot wire connected. Turn on the breaker and test continuity between the back of the breaker and the neutral bus. Since the breaker is tripping, you should see continuity/low resistance. If you do not, it's possible the short only exhibits under higher voltage but more likely, you have a bad breaker.

Assuming you do see continuity, remove neutral wires one at as time from the bus and recheck continuity. When you lose continuity, you just removed the neutral you're looking for.