I've been an aquarium hobbyist for nearly 20 years, in both fresh and marine systems, while living in Toronto, South Florida, and Tampa Bay. In that time I've dealt with a few dozen LFS, and for the past two years I've worked at one (I'm now the livestock manager).
As a hobbyist, appearance matters up to a point; I want to see a shop that is maintained (I never expected pristine, this is a messy business) and livestock that is both healthy and suitably housed. If the shop has a display tank, so much the better--that's not laid out for sales and reflects both design and husbandry ideas. The staff (from minion to owner) must be knowledgeable and at least cordial. On the knowledge side, I've learned to allow for variations across a shop's staff; I'm happy to see people who know the general stuff but also have a specialty (fish, SPS, NPS, zoas, whatever). Even now, I'm a customer from time to time (we don't have EVERYTHING) and I'm a frequent customer at wholesale/import places as well as farms. The same standards apply.
SO what are you looking for?
- are the livestock habitats clean/suitable/orderly?
- do you see pests/disease?
- is the staff knowledgeable and courteous?
- are prices reasonable? <--do not compare to private, garage-based sellers who have no overhead or taxes to pay
- will the staff tell you "no"? (not every animal is suited to every tank or hobbyist; an ethical LFS will cheerfully say "no")
As a manager, I've actually become far more demanding about where livestock is sourced and what condition it's in. Captive-bred is #1, followed by responsible and sustainable fisheries and collection agents. I likely won't tell you where we purchased the animal (unless it's ORA or ReefGen) because I've worked hard to develop a complex supply chain. I 100% will tell you (whether you ask or not, sorry!) where the animal originated, when we got it, what it's eating (here, watch it eat!), how it's behaving (oops, that firefish is a sociopath! that acro ate a monti!), and where in your tank it will succeed (if at all).
Final point: as the title says, your LFS is ALWAYS on trial. Just remember to use the guillotine sparingly.