How do you know when you have a parasite?

Depends on what parasite you have. Large intestinal parasites like Ascaris can cause diarrhea and a swollen belly. Many microscopic roundworms also cause dysentery , but can be diagnosed acurately only with a microscope. The malaria parasite causes deadly fevers. Tapeworms cause weight loss. The Filaria worms cause extreme swelling in the body- usually in legs, and sometimes in the genitals.

So many different kinds of parasites, both microscopic and macroscopic- and each has it's own complex life- cycles, hosts , and symptoms!

It's a rather gross but fascinating subject to study !