since i work at a fish store, i know what the fish look like when they come in and what to look for as far as health. But, when buying at another store, i deffinatly look for healthy fish and i throw some questions to whoever is running the fish department to test their knowledge. you may also want to get the employee to feed the fish so that you can see if they are eating or not. a good indicator for looking at fish, is to look at their stomach to see if it "caved in". good luck.