From @Humblefish
Because flukes are naturally clear a visual confirmation is nearly impossible. Confirmation of flukes can be obtained via a freshwater dip. After 3-5 minutes the fluke will become opaque and dislodge from the fish. Although helpful, it is not a necessity, to identify the fluke’s family to aid in treatment. A microscope can confirm a species, however, Neobenedenia, can be seen with the naked eye.