How do you actually verify a shop is doing genuine work and not cutting corners?

[rogelioburch]

Bit of a rant plus a genuine question here. I've been taking my C Class to the same place for two years and always felt fine about it, until a friend who knows cars looked at my engine bay and pointed out that the oil filter housing looked like it hadn't been touched in a while, despite me paying for oil and filter changes every time. I'm not saying they definitely skipped it, but now I'm paranoid and second guessing everything. It got me thinking about how little I actually know about what happens when I hand over the keys and pick the car up a few hours later. A colleague mentioned professional Mercedes Benz servicing near Musaffah Industrial Area where you can apparently ask to see the old parts or watch part of the work, which honestly never occurred to me as an option. My question is whether it's normal or rude to ask for old parts back, and whether that's even a reliable way to confirm the work was done. Also how do you build trust with a new shop without coming across as suspicious? I don't want to be that customer but I also don't want to be paying for work that isn't happening.