Lengthb returns the size of a string in bytes. To identify all the strings containing multibyte characters, check whether lengthb > length:

select * from (
  select 'àéíóú' s from dual union all
  select 'aeiou' s from dual 
)
where length(s) < lengthb(s);

S        
----------
àéíóú   

To find all the individual multibyte characters, split the strings so each character is a row. You can do this using the connect by level (or similar) row generation trick. Then a single character substr for each row. Then check the size:

select c, length(c), lengthb(c) from (
  select substr(s, level, 1) c
  from (
    select 'àéíóúbcdef' s from dual
  )
  connect by level <= length(s)
)
where lengthb(c) > 1;

C     LENGTH(C) LENGTHB(C)
---- ---------- ----------
à             1          2
é             1          2
í             1          2
ó             1          2
ú             1          2