How to recognize obsolete WAL logs

Hi all,
I participate on realization of warm standby pg 8.2.3, according to
documentation. Mostly all works fine, but one problem remains. The
recovering rdbms eats transfered wal logs fluently, but I cannot see the
way how to recognize if the recovered wal log file is really processed
into db and, consequently, is obsolete and does not need to be archived
for this recovery. Specially, when recovering rdbms is stopped, some wal
log can be processed (and automatically deleted), but the rdbms asks the
same file after restart in recovery mode. Is it some way how to
recognize such logs?
I know that I should keep base backup AND all wal logs after
pg_start_backup, but my question is NOT about safe archiving, but about
log shipping.

Jan