Is there an Oracle function to check for valid Japanese characters (Non-ASCII)?
I think the answer given the specifics is "no there is not."  I would think, however, in Oracle you could use a regular expression perhaps to find fields with characters not in a specific set.  See http://docs.oracle.com/cd/B19306_01/appdev.102/b14251/adfns_regexp.htm

I don't know how regex usage works with indexes on Oracle however.