Because I listened to a BBC radio 2 doc on that date with the self-explanatory title, 'Punk Rock USA', and I was just 'getting into' American punk then, the only stuff I'd really heard up till then was the likes of Blondie and Richard Hell and Television and the Ramones, and The Dead Kennedys. But that was the first time I heard The Stooges, as opposed to Iggy's solo stuff, mind you I hadn't really heard that either, just the songs 'Lust for Life' and 'The Passenger'. This doc explored all the proto-punk stuff, garage bands of the 60s and all, and then all of a sudden, the opening notes of '1969' started up, and needless to say, life was never the same again.
That's how I know.