Are ciguatera toxins visible

There is no visual test for the presence or absence of ciguatera. Old wive's tales used to say you could boil a silver coin with the fish in question and, if it turned black, ciguatera was present. This however there is no proven indication that this works. Another more accurate test was, if the fish was "Longer than your arm; long enough to do you harm." This probably was more accurate in that larger fish may have had a chance to concentrate more ciguatera in their systems, but again, there is no proof of this method either.

Ciguatera is known to usually appear in high end, top-of-the-food-chain predators, and is assumed to be a cumulative that concentrates near the top of the food chain, originating in dinoflagellates.

Fish taken from the same location where ciguatera was found may or may not be positive for ciguatera.

In summary, there's no test for ciguatera that doesn't destroy the meat.