How do you know it's a counterfeit 1804 liberty silver dollar?

Check the weight. All 1804 silver dollars are extremely rare, but a genuine one will have a specific weight. A class 1 will weigh 26.96 grams, class 2 is 24.711 grams, and class 3 is between 27.15 and 27.41 grams. The only authentic class 2 coin is owned by the Smithsonian.