For me, the leading cause for thinking someone is doing something shady is from past experience. That's why I pay my mechanic pretty much whatever he says. He's always been right on the nose with diagnostics and what's really unusual, is every repair for the past 15 years has never failed. I mean that's unreal, most people make mistakes sometimes or a part is bad, etc. I tip him banana bread now.
The things I've seen in the past make me pretty untrusting of mechanics. Example: once I brought a car in for a new alternator. They started taking the battery out and I asked them why, they said their procedure is to just check that first. Humm OK... Well, they said yes, you need a new alternator. So they put one in, and I was in the garage and the guy pointed it out to me under the hood (you see, I am interested!). I looked at it and humm, I asked is it supposed to have that hole in the side with all the wires showing? He said he had put in a rebuild and he had not seen that. So OK, they get a another one and put that in. I was waking back to the garage to see if it was ready and they said yes. A kid that was a helper was putting the battery back in at that point too. Would have thought they'd have done that sooner. They started the car and the battery was hooked up wrong and smoke was everywhere. So they gave me a ride home and told me to call after noon the next day. I called and asked if my car was ready, new guy said yes, but you know you have an electrical problem? I mentioned the battery issue and the guy did not believe me. Finally he called the guy that had been there when it happened.
Or maybe the tire guy, tires are a big issue. I have learned things the hard way. I was driving to Texas and needed new tires; I knew nothing about tires. Went in and pretty much told the guy just wanted nothing real expensive. He put some on, I drove to Texas and back. Winter was upon us so real quick I had my studded tires put on. Next spring, I went to have my new tires put back on (less than 3,000 miles on them). I happened to go back to where I bought them. They would not put them on. Why? Because it was unsafe, they were too small for the car. Refund? Nope. I learned what a tire size was.
I could go on, but you get the drift. My dad was an aircraft mechanic and kept our cars up. Great guy, just wish he had taught me some things about engines. He did not do that, not even how to change a tire. I have no idea why, but once he gave me his old battery charger. Probably one of the best used gifts I ever got! Oh, the Blazer was his!
I think car manuals list what needs to be done at say 30,000 miles, 50,000 miles, etc. That gets a little overwhelming when someone doesn't know as much about maintenance. If someone made a list like oil changes every 3,000; air filter every year; rotate and balance every 3,000; plug wires etc. It would make it so much easier to understand and track! If a shop drew up a tracking sheet to give to customers to track their mileage or dates on they'd love you for that and would not neglect maintenance as much. I have designed forms for a living, I am going to design one of these for me! It would help me learn too.
Take care and thanks again!