Here in Pennsylvania, assignments are indexed by borrower, lender (assignor) and assignee.  Most, if not all the county recorders require the instrument to state the instrument number (or volume & page) and recording date of the mortgage to which it relates.

When instruments were kept in books "back in the day", it was custom and practice for Recorders of Deeds to reference the volume and page of the assignment right in the volume, on the face page of the mortgage.  Same with satisfaction pieces.  Fortunately, that custom has carried over to electronic media in this state.  In some counties, the assignment/satisfaction info is added directly to the indexed entry; for others, it's a matter of simply seaarching all indexed entries under a given borrower's name.  Hope this helps!

Regards,
Scott Perry