Check if table exists

Even if I've been using Postgres for some years, I figure that this
question fits best on this list:

How do I check if a table exists, eg. from a PLPGSQL function?

I've got a section of a function that only should be run if it's called
from a certain context, that is if the temporary table 'tmp_sources' is
found. But if I try with the line

IF EXISTS (SELECT * FROM tmp_sources) THEN

The function throws an error:

ERROR: relation "tmp_sources" does not exist

Which is kind of off the mark in my context.
--
Leif Biberg Kristensen | Registered Linux User #338009
Me And My Database: http://solumslekt.org/blog/