Location of database files?

Is there any way to determine the location of files in a database
without being the postgres user? Essentially i'm after the setting of
PGDATA so i can then show disk status (df) for that partition.

The pg_database catalogue has 'datpath':

If the database is stored at an alternative location then this
records the location. It's either an environment variable name or an
absolute path, depending how it was entered.

so I'm really looking for the default location...

I could knock together a C function to do this (and indeed another to
return the usage stats too), but would like to check first there's no
simple way already!

Regards, Lee Kindness.