If you run a select statement and get the following error…

"SQLSTATE 01517: A character that could not be converted was 
replaced with a substitute character."

… you can use the TRANSLATE function to strip away printable chars, and compare that to a zero length string like so…

SELECT * 
FROM XX.TBL 
WHERE TRANSLATE(UPPER(FIELD_X),'','ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890!@#$%^&*()-=+/\{}[];:.,<>? ') <> '' 
FOR READ ONLY WITH UR;