Testing if a Column Exists in a NEW record of a Trigger Function

Dear List,

As part of the Audit process in my psql database, I have a standard set of columns "InsertBy", "InsertTime", "LastUpdateBy" & "LastUpdateTme".  I use a standard Trigger function [listed below] to ensure the correct details are added to each record. 
However, on some data tables where data is only imported and not updated, the "Update" columns are not required.

As such, is there a way to test the NEW record variable to see if the column "LastUpdateBy" exists?   I could not find any information in the manual to describe how this might be done.

Thanks
Simon

CREATE OR REPLACE FUNCTION "Security"."InsertStamp"() RETURNS trigger as $InsertStamp$
    DECLARE
               UserID integer;
    BEGIN
                SELECT "IDUser" INTO UserID FROM "Security"."User" WHERE "Username" = current_user;
                NEW."InsertBy" := UserID;
                NEW."InsertTime" := current_timestamp;
                NEW."LastUpdateBy" := UserID;
                NEW."LastUpdateTime" := current_timestamp;
                RETURN NEW;
    END;
$InsertStamp$ LANGUAGE plpgsql STABLE SECURITY DEFINER;