Most kosher mistakes at the fish counter come from names, not biology. A few patterns cause the trouble, and once you know them you can sidestep most of them.
Look-alike swaps
A cheaper non-kosher fish can be sold as a pricier kosher one, especially as a fillet where the skin and scales are gone. White fish fillets are the easiest to mix up. When the price seems too good, check the species and look at the warnings on that fish page.
Loose market names
Names like “snapper,” “bass,” and “sole” get used for several different fish, and not all of them are the same species. Search the exact name on the label and read the nicknames section to see what it might really be.
Sushi surprises
Sushi menus include non-kosher items that sit right next to kosher ones. Unagi is eel and is not kosher. Surimi and “crab” sticks are processed and need a closer look. Roe and caviar depend entirely on the fish they come from.
Skinless fillets
Once the skin is off, you cannot see the scales, which is the whole test. That is why a reliable source for the species matters, and why some authorities prefer fish sold with a patch of skin and scales still attached.
For any of these, the fish page warnings section calls out the specific risk. When a purchase really matters, ask a trusted rabbi.