Wow sad, even those mechanics which I visit regularly and 'trust', I try to be there all the time. And of course one has to know the ABC of cars, otherwise simply being there would not help. For example if I go for some specific issue and I see them touching something which is not related to that issue, I will ask them why are you doing this, and then see if their response makes sense to me.
The other thing which always alarms me is when I see mechanics puzzled, looking around - that takes all the confidence away.