plpgsql/rule question

Is it possible to tell if a column in a NEW record in a plpgsql function was
explicitly specified as NULL or simply left out altogether?

For example, if I have the following table:

    create table foo(id serial, msg varchar)

Is it possible to distinguish within plpgsql between these two queries?

    insert into foo (msg) values ('Hello')
    insert into foo (id, msg) values (NULL, 'Hello')

TIA.

Ed