How to identify non utf8 symbols in a range of text?

In our database we have Latin and Cyrillic alphabets and all is fine with those characters. But, sometimes people enter characters that appear as ? or squares. Is there a way to find those specific symbols, because looking manually through a couple of tables with >400 000 records for 1 mistake is insane.

I have looked for online solutions that I can load my data as text, I have tried notepad++, but either I get all Cyrillic marked or I don't get anything.

EDIT: L.􀈉.D. is an example.

Based on the ASCII value of the character you can filter the text.Here i am assuming all characters greater than 65534 as outside the normal range.You can modify the range according to your requirement.The db fiddle here

 with test (col) as (
  select 
    'L.A.D' 
  from 
    dual 
  union all 
  select 
    'L.􀈉.D.' 
  from 
    dual
) 
select 
  col, 
  case when max(ascii_of_one_character) >= 65535 then 'NOT OK' else 'OK' end result 
from 
  (
    select 
      col, 
      substr(col, column_value, 1) one_character, 
      ascii(
        substr(col, column_value, 1)
      ) ascii_of_one_character 
    from 
      test cross 
      join table(
        cast(
          multiset(
            select 
              level 
            from 
              dual connect by level <= length(col)
          ) as sys.odcinumberlist
        )
      )
  ) 
group by 
  col
  having max(ascii_of_one_character) >= 65535;