When the fish lice are young they appear as very small dark spots and unless you see them move sometimes they are overlooked. They move very fast and the best place to look for them is on the fins because they show up better on a transparent background. (Please see pictures in this article)
Koi and pond fish with fish lice will “Flash” rub against the sides, bottom and the rocks of your pond, some will show signs of red lesions on their bodies. Anchor Worm and Fish Lice are the only common fish parasite you can see with the naked eye.