I can't speak for the US but a friend in the UK was recently diagnosed with giardiasis, not chronic though. As far as I can tell it was simply a routine check because of the symptoms. He didn't give me the impression that it was a difficult diagnosis to obtain. The medics reckoned that it came from a bag of contaminated salad leaves.

Surely you just go to your GP, explain your concerns and symptoms and get tested. Here is the UK NHS page on the subject: https://www.nhs.uk/conditions/giardiasis/