Gill Flukes cannot be seen by the naked eye. You would need to do a swab and look at it under a microscope to see the parasites.
Here's a link that shows how they look under a miscroscope, http://www.fishdoc.co.uk/disease/flukes.htm
"The fish response to the irritation caused by flukes is similar to any other irritant, whether it is parasites or chemical. Initially there will be rubbing and flashing. As the disease reaches a more advanced stage the fish will become lethargic - which is the point when some people think the problem has gone away. At a very advanced stage the fish will isolate itself and spend long periods laying on the bottom with its fins clamped to its body. Other signs may be skin cloudiness resulting from excess mucus production, skin hyperplasia, or focal reddening. A definite diagnosis can only be made via a skin scrape or gill biopsy. In large numbers, flukes will kill fish either directly, or indirectly through secondary infections."
Adding salt during the Prazi treatment will help prevent secondary infections. HTH
