derive the sequence name of a column

Hi!

Can anyone please tell me exactly how the name of a sequence a field is
using (manually created or generated by a serial) could be derived
programmatically (using libpq or through SQL)?

Thus:

CREATE SEQUENCE an_unknown_sequence_name;
CREATE TABLE test (
test_id integer default nextval('an_unknown_sequence_name'),
useless_redundant_test_id serial
);

How do I programmatically extract that column 'test_id' in table 'test'
is using 'an_unknown_sequence_name'

TIA!

Mel