I just went on fb groups for my local neighborhood - highest upvoted pick I chose. That was 4 cars ago still take my cars there. Was a dealership user before that myself.

I knew he was trustworthy because I asked him to explain what he was doing and why. He went into great detail. Brought an unidentifiable problem to him and he found a corroded wire vs 700 in repairs the dealer suggested. Just little things like that will let you know. Check their certificates, make sure at least ASCE (sp) certified.

Also take a look at the shop and see what cars are in the lot. Bunch of 5-10 year old cars in great shape seems to be a good sign IMO as people who take care of their cars trust the mechanic