I think you approached this situation really well - especially since you spoke to the developer and asked: “How am I supposed to test this?” It seems to me, you worked on the assumption that the change was testable, not that it might or might not be testable.
How I would’ve approached it: (to be honest, it’s pretty similar to the way you outlined it)
- If the changes are not controlled or observed via the UI, I would ask myself, and then the developer where can I see the changes? How can I control the changes? How do I know the changes are correct?
- If there is any lack of feedback, I would ask for the logging to be improved and give them some examples of what I would expect and why. I prefer that developers feel ownership over their code, so I ask questions and make suggestions, but they would make the decisions on how to implement it.
- Lastly, I ask them are there any other applications (current or upcoming) we may need to change or update now that we have learned about this together.