That's the thing, though. If he's within a reasonable range, then ok. If he's way outside that range you should take a pause and consult someone else. It's just a sanity check on the mechanic, like calling a friend or another shop.

I had a great mechanic for a while who I could trust, but at some point he retired and sold his shop to someone new. I ended up with a problem that seemed to be getting into the thousands of dollars range. Is he right? Is he lying/taking me for a ride? Is he being lazy? I'm a pretty technical guy, but I don't know everything about the ins and outs of auto mechanics.

I told him to wait, and after a few calls to car-friends and a dealership I could call him back and have him take me through the problem again. My goal wasn't to 'trap' him, but now that I knew enough I could see what he looked at, what he thought the problem/options were, etc.

I ended up taking the car to a different mechanic, not because the first was lying to me, but because it was clear that he didn't have enough knowledge about my type of car. It ended up being a cheap part and a quirk with the car's computer.

I later told the other mechanic (in a friendly manner) and he was genuinely surprised, and offered me a discount in the future if I ever needed anything else.

A customer waiving their iPhone around is probably a pretty good way to show that they don't know anything about car repair and are bad at negotiating. That's probably not going to work out so well for them...