Just got to a new location and visited the dive shop I'm planning to use. My first criterion was that they support tech diving, as I do tech diving, and they are likely going to be more knowledgeable than a recreational-only shop.
They mix trimix, they don't push you to buy anything, they have a pool on-site with a 10' deep end. They don't just stock one brand, and they don't just stock the expensive stuff, but have a variety of choices. I love when I see a bunch of odds and ends made by Trident in a shop because I know they care about what works more than what costs the most. They have a couple of scooters sitting around. They have a Petrel in the display case. They stock side-mount and back-mount gear, lots of different sizes of clips, pouches, pockets, backplates, etc. for sale. I also love when I see lead weight for sale, because IMO they are more versatile than the bean bags. And their price sheet for fills, visual inspections, O2 cleaning, etc. are reasonable.
And when you talk to the instructors they know their theory and good practical info.
That's how I know I'm in a good shop.