The only time you can trust an LFS is if they are a true fish only store. They are few and far between, but if you are lucky to have one I would consider developing a relationship with them. My LFS only sells fresh and saltwater fish and supplies, everyone who works their keeps at least 4 tanks and are willing to say "come in tomorrow when bob is here. He is our big reefer." or something like that if you have a specific question. I also just recently found out they will price match online prices for at least the supplies (not sure about livestock).
Anyway, I did not mean to ramble. I just wanted you to know that there are good LFS out there, but if they also sell dog bowls and parrots they probably know little to nothing about fish.