serial column detection

Hi!,
how detects PgAdmin when it should show a column as "type" serial? i am trying
to replicate in a existing table the conditions to get shown as serial but i
don't get it.

I had a table like this:

CREATE TABLE test
(
test_id integer NOT NULL,
CONSTRAINT test_pkey PRIMARY KEY (test_id),
);

that i would like to see as:

CREATE TABLE test
(
test_id serial NOT NULL,
CONSTRAINT test_pkey PRIMARY KEY (test_id),
);

i have tried with:

CREATE SEQUENCE test_test_id_seq;
ALTER TABLE test ALTER COLUMN test_id SET DEFAULT
nextval('test_test_id_seq'::regclass);
ALTER SEQUENCE test_test_id_seq OWNED BY test.test_id;

and now i have this table:

CREATE TABLE test
(
test_id integer NOT NULL DEFAULT nextval('test_test_id_seq'::regclass),
CONSTRAINT test_pkey PRIMARY KEY (test_id),
);

What should i do? Thanks!

Regards,
Miguel Angel.