Streaming Recovery - Automated Monitoring

I'm trying to come up with an automated monitoring system to watch the
WAL log progress and sound appropriate alarms if it gets too far behind
for some reason (e.g. communications problems, etc.) - so far without
success.

What I need is some sort of way to compute a difference between the
master and slave(s) position in the WAL stream. It appears that there
is no function set specifically to do this, and one of the things I
thought I'd try (using pg_xlogfile_name_offset) doesn't work on the
slave - it returns:

ticker=# select pg_xlogfile_name_offset(pg_last_xlog_receive_location());
ERROR: recovery is in progress
HINT: pg_xlogfile_name_offset() cannot be executed during recovery.

Any ideas?

Looking in on the system is ok, but I want to come up with an automated
tool for letting me know if something goes haywire.

Thanks in advance.

-- Karl