It has to come down to trust. I trust my mechanic. He has treated me well since the beginning. The first thing he ever did for me was assemble the coilovers for my KJ. I supplied all the parts except the big-ass washer that goes on the end. He even asked me if I was sure all the parts were there and I was sure they were. I was wrong.
The mechanic I was using to do the lift took the coilovers back to him, his competitor (I went to him because he had a spring compressor and my mechanic didn't). He disassembled the coilovers, added the washer and reassembled them. When I tried to pay him, he refused.
Then I started going to him regularly. My original mechanic was a great guy, but he worked for a guy I was having difficulties with, so I moved on. Now my new mechanic has done a number of jobs, both repair type and modifications type and always treated me fairly. He would prefer to supply all the parts, but doesn't complain when I buy them myself. I don't think he has ever billed me book hours, almost always less. Even though it's prob'ly against the law, he allows me to be inside his shop to take pictures or just to chat.
I don't know what he did to discover the motor needed to be replaced, but he is very methodical and you have never seen a shop as clean and organized as his... well, I haven't. But to answer your question, no, I'm not sure, but he is. I trust him as much as I would, say, tommudd - and I don't even like Tom as much as I like my mechanic, but I trust him. That's all I need.