How did everyone posting here find out they had a parasite problem...?"
"If you have any one or more of the above symptoms (see website), or if you have reason to believe that you may be an asymptomatic patient by reason of foreign travel, infected household contact, bad meal, etc., you should consider being tested for parasites." (Parasitology Center Inc is one of the premier U.S. gastro infection labs.) http://www.myplanet.net/yeec60bp/understand_parasites.htm
The gastro infection rate is one-in-five according to the book "Restoring Your Digestive Health," written by two gastro specialist MDs. Other statistics from both the traditional and alternative medical communities are in-line with these estimates.
"...or does everyone have them to some degree?"
To my knowledge, only the Hulda Clark organization, which sells infection treatments as their primary profit center, claims that "everyone" is infected.