Here is a
link to an IRS pub that might shed some light on the subject. As a practical matter, if you reside in a country where the IRS would have any chance of collecting on US taxes, you probably have a tax treaty with the US that is reasonable. British subjects after Brexit, well, whatever...
Substantial Presence Test You will be considered a U.S. resident for tax purposes if you meet the substantial presence test for calendar year 2018. To meet this test, you must be physically present in the United States on at least: 1. 31 days during 2018, and 2. 183 days during the 3-year period that includes 2018, 2017, and 2016, counting: a. All the days you were present in 2018, and b. 1/3 of the days you were present in 2017, and c. 1/6 of the days you were present in 2016.
Even if you meet the substantial presence test, you can be treated as a nonresident alien if you: • Are present in the United States for less than 183 days during the year, • Maintain a tax home in a foreign country during the year, and • Have a closer connection during the year to one foreign country in which you have a tax home than to the United States (unless you have a closer connection to two foreign countries, discussed next).