RMAN list archivelogs that are needed for to recover specified backup
Depending on your requirements and what other backups you have done since then, you could use (from http://docs.oracle.com/cd/B28359_01/backup.111/b28270/rcmreprt.htm#BRADV89594):

REPORT NEED BACKUP RECOVERY WINDOW OF n DAYS
Displays objects requiring backup to satisfy a recovery window-based retention policy.

REPORT NEED BACKUP REDUNDANCY n
Displays objects requiring backup to satisfy a redundancy-based retention policy.

REPORT NEED BACKUP DAYS n
Displays files that require more than n days' worth of archived redo log files for recovery.

REPORT NEED BACKUP INCREMENTAL n
Displays files that require application of more than n incremental backups for recovery.


These will tell you which files you need to backup to provide 'n' days worth of restore capability or 'n' redundancy.