Finding out free space?

I need to provide an alarm function that will alert my customer when the
amount of free space in a database has reached some critical threshold. This
problem is not entirely trivial, since free space can be located both in the
file system and in the database itself.

I'm not looking for a particularly accurate function (and suspect that one
is probably not feasible anyway) - anything that comes to within a gigabyte
of the actual value is fine with me. Basically I need to figure out two
things:

1. Where does Postgres stores its datafiles? This will allow me to figure
out how much space is still available on the filing system.

2. How much space is unused within the data files themselves?

I'd appreciate a pointer on where I should look for this sort of
information.

Regards,

Hans Guijt