External parasites will often leave marks- like white spots from ich. or flashing/scraping on ornaments
Intestinal parasites will show signs like white stringy feces, not eating for long bouts of time, or with camallanus sometimes red worms hanging out of the fish's anus are visible.
Other parasites, like gill flukes can result in flashing/scraping behavior as well as rapid breathing.
what makes you think the fish might be sick?