How to find if a function is inbuilt function or user-defined in postgres

Question

I would like to know if a function is inbuilt or its a user defined function in postgres. I have tried writing the query below, but there is nothing that tells me if its either or of the two. I took this query from postgres logs after i searched for a function in pgadmin. The result list also show functions like dblink.

 SELECT obj_type, function_owner,obj_name,
            pg_catalog.REPLACE(obj_path, '/'||sn.schema_name||'/', '/'||CASE sn.schema_name
            WHEN 'pg_catalog' THEN 'PostgreSQL Catalog (pg_catalog)'
            WHEN 'pgagent' THEN 'pgAgent Job Scheduler (pgagent)'
            WHEN 'information_schema' THEN 'ANSI (information_schema)'
            ELSE sn.schema_name
            END||'/') AS obj_path,
            schema_name, show_node, other_info,
            CASE
                WHEN sn.schema_name IN ('pg_catalog', 'pgagent', 'information_schema') THEN
                    CASE WHEN CASE
            WHEN sn.schema_name = ANY('{information_schema}')
                THEN false
            ELSE true END THEN 'D' ELSE 'O' END
                ELSE 'N'
            END AS catalog_level
        FROM (
            SELECT
            proowner::regrole::text function_owner,
                CASE
                WHEN t.typname IN ('trigger', 'event_trigger') THEN 'trigger_function'
                ELSE 'function' END::text AS obj_type, p.proname AS obj_name,
            ':schema.'|| n.oid || ':/' || n.nspname || '/' || case when t.typname = 'trigger' then ':trigger_function.' else ':function.' end || p.oid ||':/' || p.proname AS obj_path, n.nspname AS schema_name
,
            CASE WHEN t.typname IN ('trigger', 'event_trigger') THEN True ELSE True END AS show_node,
            pg_catalog.pg_get_function_identity_arguments(p.oid) AS other_info
            from pg_catalog.pg_proc p
            left join pg_catalog.pg_namespace n on p.pronamespace = n.oid
            left join pg_catalog.pg_type t on p.prorettype = t.oid
            WHERE (CASE
            WHEN n.nspname = ANY('{information_schema}')
                THEN false
            ELSE true END) AND NOT p.proisagg



        ) sn
        where 1=1 --lower(sn.obj_name) like '%test12345%'
        AND NOT (sn.schema_name IN ('pg_catalog', 'pgagent', 'information_schema'))
        AND (sn.schema_name IS NOT NULL AND sn.schema_name NOT LIKE 'pg\_%')
        and function_owner <> 'appuser'
        ORDER BY 1, 2, 3