How to find all activities generated by one schedule reminder?
The easy answer is to search by the email's subject line - but I'm assuming you have multiple scheduled reminders with the same subject line.

You can find details in the civicrm_action_log table.  The action_schedule_id is the ID of the scheduled reminder.  That said, I can't 100% recall how this handles sending the same reminder multiple times - either because repetition is enabled, or because the same membership comes up for renewal multiple times.  Hopefully neither of those apply to you, but if they do, you may need to compare that data against the activity log to see how it's represented.