Ultrasound would show enlarged bile ducts, not the flukes.
The definitive test is to look for the eggs in the stool sample. One adult fluke lays 2000-4000 eggs each day and the eggs are excreted through the bile ducts and feces of the infected person.
If liver fluke is left undetected for years, this can become a very serious disease and cause destruction of the liver. Most people with liver fluke infection are completely unaware because they experience no symptoms at all. A small percentage of patients experience fatigue and non-specific abdominal discomfort, which easily gets mistaken for indigestion or irritable bowel syndrome.
Flukes are very hard to eradicate. It can be done but often takes a long time depending on the amount of flukes in the liver.