As per AC39-9, FAA recommended the support element, namely engineering staff should ensure that AD method of compliance is concisely tracked and recorded. How engineering staff in the office can be sure of adherence of mechanics to the method of compliance when they are performing inspection/modification?
The mechanic's signature or stamp is a statement of compliance. If the item is deemed important enough, then usually a QC follow-up assures the work has been done correctly.
All work must be done to Maintenance Manual, Structures Manual, or Wiring Manual directions. If any modifications are done to an aircraft.... they must reference these manuals and the procedures therein.
You could physically observe at any time, but the paperwork is always the main driver.
That is not an easy question to answer.
What are we talking here? 20 aircraft? 50? 100?. What systems? What vendors?
You would have to do hours of work just on one item.
There are qualified record investigators out there. If you think the need is important..... I would hire a third party for all that work.