unmatched query
Use an Unmatched Query to give you a list of records that don't match up.
For a one-to-many relationship, I usually list the many side. Most often, these become Orphan records -- meaning they don't have a Parent Record on the one side on which to be matched. Unfortunately, quite often there is little that can be done with them, unless you have a copy of the database with the missing record(s).
In my case, I had Orphan records because I had not enforced Referential Integrity initially with Cascade Update and Delete. Lesson learned.