> I think we could inspect chunks of iron containing mudstone deposited anytime in the past few million years. By looking at the magnetic alignment of iron particulates, you will learn the magnetic field when the particle was laid down.

This method has been used to identify the reversal of the magnetic pole. [1]

Generally tho, such solar storm lasts for way too short amount of time to leave permanent evidence in the rock layers. However, I believe they have studied deviations in tree trunk rings.

1: https://en.wikipedia.org/wiki/Geomagnetic_reversal