One tip for using pet shops, do a little research about what your looking at. Then a few standard questions and when they are relaxed an chatting drop a few questions that may be species specific.
Then see if they stumble then admit a lack of knowledge or just run straight into a diatribe of complete tripe.
As example, this afternoon i picked up some bamboo shrimp, they had them listed as wood shrimp. I asked for the bamboo shrimp, they had amano's and cherries. She went straight to the correct tank, and commented (quietly) on them being hungry as they are nearly impossible to properly feed in the shop.
Not iron clad guarantee of deep knowledge, but she definitely understood the stock she had.