(replication) Detecting if server a slave, or a master in recovery

Hi,
What is the correct way to tell what mode a replicated Pg server is
currently in, via a database connection?

I can take a guess if it's the master or a slave by using the
pg_current_xlog_location() and pg_last_xlog_replay_location() functions.

However it occurs to me that a master server, which has been roughly
rebooted and is replaying its WAL files will probably give the same
response as a slave, when I call the current_xlog_location().


What is the best method for determining whether you're connected to the
master or slave database?


Thanks,
Toby