How to recognize obsolete WAL logs

[Jan Poslusny <pajout@xxxxxxxxxxxx>]

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into db and, consequently, is obsolete and does not need to be archived 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