I'd say have a look at the general state of the tanks / fish - look at there general health etc; labeling of fish, what type of things they sell, e.g. v. small tanks for betta's; if they sell dyed fish, etc.
Then, I'd ask them some advice on a fish that you claim to be interested in keeping, but that you KNOW would be unsuitable for your set up - e.g. an oscar cichlid in a small tank with neon tetras, or keeping just one shoaling fish on its own, or whatever. And see what advice they give you. It might be better to avoid asking "controversial" questions, or ones where you might expect differences of opinion.
You could also ask more clear-cut things like how big something will grow, the temparature range it needs, etc, and check what they tell you against other information sources.