How do I know that I am a US person?

As a private individual, you are a US Person if you are considered a US citizen/national or a US residential alien. The term “US citizen/national" comprises:

Any individual born in the United states (including Puerto Rico, Guam, U.S. Virgin Islands, American Samoa and the Northern Mariana Islands);
Any individual whose parent(s) is a US citizen (additional conditions apply):
Any individual who has been naturalized as a US citizen.
The term "US resident aliens" comprises:

Any individual that possesses or has possessed a US green card.
Any individual that meets the substantial presence test for the calendar year.
Please visit www.irs.gov for more information about the green card and the substantial presence test.