finding non ascii characters in the database

We're having some issues with ascii characters when migrating tables over to Oracle. What would be the best way to determine which tables and columns contain non-ascii data ? All I can think of now is to either unload the whole database and do a unix od command or some other grep for non-ascii characters, or some query to select all rows of all tables with a where clause that selects non ascii characters. any ideas would be appreciated. Thanks, floyd