How do universities verify your native language?
Actually, there are no universal requirements for waiving testing. For instance, at MIT, you establish eligibility for a language waiver by attending primary and secondary schools that have English as the language of instruction or by attending an American university for your undergraduate education. At Yale, you must have attended an English-language university for at least three years to establish eligibility. Ohio State only grants exemptions for students graduating from universities in Australia, Belize, the British Caribbean and British West Indies, Canada (except Québec), England, Guyana, Ireland, Liberia, New Zealand, Scotland, the United States and Wales.

So this unfortunately becomes an example of "every school is different, and you need to check the rules for each school."