If they tell you something needs fixed/replaced, ask them to show you.
A reputable shop will have no problem showing you the issue.
A good mechanic will be able to explain in relatable terms why it’s a problem.
Any shop that can’t sit and explain what’s wrong and why to fix it is either full of shit or incompetent.
Also, you are legally allowed to ask for old broken parts back. Had your brake pads replaced? You can ask for the old ones. The only thing might be that a lot of parts have a core charge so asking to keeps the parts might raise the cost of your service.
EDIT: a core charge is sort of like a deposit. For example, you buy a starter motor, it’s $100 and has a $10 core charge. If you bring back the old starter motor then the store will refund you $10. Then the store sends your old motor to the manufacturer so they can rebuild it or recycle some parts. Sort of like 5 cent deposits on aluminum cans in some states.