Simple things like going for a replacement tyre and being given a choice of price ranges and an explanation of each option, then watching them do the change and hand torque to the correct spec.
Or an air con regas at £50 at a small garage vs £120 at the main dealer.
Most recently I had the discs changed, was able to order the parts myself and drop them off for fitting at the cost of labour only. That'd never be an option at a main dealer or chain type garage but is pretty common for smaller outfits apparently.
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
Very much easier said than done for most people. Assuming you aren’t lucky enough to have a mechanic in your circle, how do you tell whether they are trustworthy?