If they ask you questions and aren't afraid to tell you 'no', that is a step in the right direction.
Look at what tools they use and how they care for their own tanks. If they use strips to test (bad) or a hydrometer (bad) versus bottled test components and refractometer (good). If have fish with ich in display tanks, that is a sign they don't take very good care of their stock. How is the variety? Do they have only damsels and clowns (bad) or do they have higher end fish for sale that look healthy, such as tangs? Do they have good LPS/SPS corals, or just a lot of softies, which tend to be easier to keep? What do they feed their fish (pellets only would be bad whereas a mix of live and fresh seafood would be good)? Ask them how to cycle a tank? If they say to throw a damsel in, walk out. Ask questions you know the answers to just to see how they answer. i.e. I am looking for a small Pacific Blue Tang to add to my 3" yellow tang in a 29g biocube. See if they'll sell it to you. If they do, walk out. Tell them you just put in the salt, LS, and LR yesterday on a new tank and want to add some fish. If they offer to sell it to you, walk out.
Listen to other conversations with other customers. Can they educate their customers, or just sell.
Ask for a sample of their water to test. Test the parameters you normally would and see if their water is in line with recommended ranges.
Ask what their process/policy is for getting new fish in stock through selling them. Some will take them in and sell them out the door same day. Others will wait about two weeks, or until they know they are eating, etc.
Will they hold fish until they see it eating before selling? Good sign.
Bottom line, they should be able to care for their tanks, educate customers, and not be afraid to say "no" to a potential customer. The smart hobbyist, who is in it for the long haul, will be back to the LFS they can trust. This hobby is too expensive to throw money away....at least for most of us.