How do we tell?

If they weren't shouting their intentions, and refused to answer when asked, then I'd agree with you, that it's difficult to tell.

However in this case, it's quite easy to determine whether they're engaged in sedition.

The sedition charge has legal criteria that must be satisfied. Jurisprudence has validated each step of the prosecution theory for seditious conspiracy.

That's literally COMMON LAW

If you don't like our western common law, then you could always try moving to the third world

https://uscode.house.gov/view.xhtml?req=granuleid:USC-1999-title18-section2384&num=0&edition=1999#:~:text=If%20two%20or%20more%20persons,force%20to%20prevent%2C%20hinder%2C%20or

If two or more persons in any State or Territory, or in any place subject to the jurisdiction of the United States, conspire to overthrow, put down, or to destroy by force the Government of the United States, or to levy war against them, or to oppose by force the authority thereof, or by force to prevent, hinder, or delay the execution of any law of the United States, or by force to seize, take, or possess any property of the United States contrary to the authority thereof, they shall each be fined under this title or imprisoned not more than twenty years, or both.