Unless it's something that can be immediately seen, they'll usually say something like: "Well I won't know until I open it up"
Best thing to do is ask them for the old parts back, as evidence that they were replaced.
I'm glad I learnt about cars from a young age, and perform pretty much all repairs on my cars myself. I not only save a stack og cash, I also get to miss out on the bs mechanics spill.
Fair enough. But still would require you actually knowing what they were. But I again it would be a good tactic....they don't know what you do or don't know.
I have a mechanic in my family. He is not dishonest. But sometimes if an issue is complicated, they are sort of guessing (educated guessing) and may end up replacing something that was not needed to be replaced or they find much bigger problems after they get into the project. Now a good mechanic will stop and call you first to get the okay to continue.
I have run into this twice. Once one was looking at my tie rods and called the other over who did not miss even a step in his stride. "Oh, yeah, that baby is loose, it is gonna fall off" Then I got the story about I cannot pass it for inspection because you got kids and this is a safety issue. I told him I would have to talk to me husband. We took it somewhere else and it passed just fine. I should have took it to the highway patrol and had them look at it.
Another was repairing the front end and called saying they were having a hard time finding the parts. But the did and they were higher. OK. Then they called and said they couldn't get those but they found some at a junk yard and would it be ok to use them. I said fine. So we go to pick it up and the price it even higher than before. I asked about the used parts and the guy looked funny. He pressed some keys and the camera down to half what they were gonna charge. We never went back to either. If possible you should call around and get quotes. And check their feedback. Ask neighbors for where they go, and once you find a good honest one stay with them.