You obviously can't see the parasites themselves, but one of the more obvious signs is white, stringy feces. When I was dealing with them several years ago in some rescue guppies, I also noticed the abdomen could appear bloated as well, although this may not be the case in all situations. Keep in mind that I haven't researched this in several years though, so there may be some new manners of diagnosis that I am not aware of.
As far as treatment is concerned, I found that only the feed-through medications were actually effective.
Jungle fizz tabs, worked for me whenever anyone had internal parasite But, the food-medication (if he'll eat) are worth getting. I don't have a picture, but there are signs...like everyone else already said.
No idea of general cure but I have some stuff from API and it's pretty good.
Internal parasites hitch hike on a host (new fish), on plants, and transfered by contaminated ornaments/fake plants, water, nets, siphons, buckets... much like diseases, especially if those items are kept wet and used frequently. It's always smart to quarantine new fish and all new plants for at least 2 weeks, because then you can tell there are or are no parasites And smart to clean objects used between tanks!