Use
select *
from all_external_tables;
to see all external tables your user as access to. To see them for a specific schema/user:
select *
from all_external_tables
where owner = 'ARTHUR';
If you only want to see the ones owned by your current user, use
select *
from user_external_tables;
To see all table that are not external tables use this:
select ut.table_name
from user_tables ut
where not exists (select 42
from user_external_tables uet
where uet.table_Name = ut.table_name);
More details in the manual: