My local fish store is not under Rabbinic supervision, and it sells fillets without skin. How could I tell if the fish they are selling are kosher?

You cannot! Even if the fish is halibut, whitefish or carp (all kosher fish), once the skin is removed it is impossible to identify, and it cannot be assumed to be kosher. In determining the kosher status of fish, identifying the species is critical. There are two ways to identify a kosher fish: • By removing a kosher scale from the skin.11 • By recognizing the fish as being from a kosher species. One can only recognize a fish species if the skin is still intact. It is generally impossible, even for a “maven”, to identify fish without skin. The exception to this rule is that the Orthodox Union accepts salmon and red trout fillets without skin, as there is no non-kosher fish whose flesh resembles that of a salmon or red trout. For example, let us say that you want to purchase tilapia.12 You heard that tilapia is a kosher fish, and the friendly counterperson assures you this scale-less fillet is tilapia. You simply cannot rely on this person, unless he is both observant in Torah and ...  more