how to tell if a replication server has stopped replicating

Hello,

We recently discovered, quite by accident, that our streaming replication server was no longer replicating. We noticed
thisin our master server log file: 
2011-08-26 00:00:05 PDT postgres 192.168.17.4 [unknown]LOG:  replication connection authorized: user=postgres
host=192.168.17.4port=53542 
2011-08-26 00:00:05 PDT postgres 192.168.17.4 [unknown]FATAL:  requested WAL segment 00000001000001D10000006B has
alreadybeen removed 

As it turned out this has been going on for at least a week as everyday's log files were crammed with these messages.
Whatevercaused the replication server to end up needing the WAL file is a mystery for another day. What I would like to
dois setup a simple method of alerting us if replication stops. We could do a simple grep of log files on the
replicationside, but I am guessing that there is some SQL command that could be run against the postgres internals that
wouldbe cleaner. Is there such an animal? 

Thank you,
Bill MacArthur