I trust our engineers. If I can, I like to chat to the engineers who worked on the aeroplane and get a feel for what was wrong and what had to be done to rectify it.
Back in the GA days if I didn't trust them* (and that was very very rare) I'd offer to take them up with me on the test flight. If they refused, I'd refuse the aircraft and tell them to fix it.
*not company engineers.