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.