One simple way to do this is to use a purpose-specific relationship. Just create a new relationship that matches the existing one, but add the two date fields as an additional predicate (Clients::Revised < Notes::Revised). A portal will reveal any records that match the criteria.
If you don't want to do that, you can run a script from the Notes table and perform a Find where the criteria are the same (Clients::Revised < Notes::Revised). That will return a found set of Notes records that correspond to the mismatch.