In most cases, you will be able to see external parasites because they will appear as spots or tube-like organisms and will continue to grow. Your fish will also begin rubbing up against plants and other decorative items in the aquarium in an attempt to rid themselves of the annoying pests.
Internal parasites are much more difficult to detect. Fish with internal parasites rarely display any behavioral problems that provide a key to diagnosis. By the time a fish with internal parasites does display unhealthy behavior, the infection has already progressed to a terminal stage.
If one of your fish does die and you suspect parasites, you can have an autopsy done by an aquatic veterinarian to diagnose the problem. Very experienced fish owners sometimes perform their own autopsies. Since you must know what you are looking for, simply viewing the internal organs of your fish is not enough. A postmortem procedure can help you save your other fish from the same fate because when you know the culprit, you can initiate the proper treatment.