From what he is and what he does really, rather than what he says. As with many larger than life characters I supect his enterprises are an extension of his psyche - PayPal for example. A lot of Ti is non-verbal, such as in the way a tennis player judges how to position themselves in a game as it unfolds (ref eg Leonore Thompson - "Personality Type, An Owner's Manual"). One theory is that it's a right brain introverted function (which it shares with Ni), so from the outside it can sometimes seem quite hidden and without an explicit flow of logic obvious to someone else. One way I experience this aspect of Ti myself is when I'm driving and Se/Ti does a lot of the work - and I don't express that thinking process as a narritive.
But for Elon this is a bit of a speculative reach on my part - I'm agnostic on his type. I'd need to experience how he acts within his businesses, more than what he says, to get a better idea of it. I can't help feeling that in his interviews he's not hitting me with his authentic self, though I don't think that's deliberate - just a feature of being in the public eye a lot with heavy media exposure and may be a not too well developed introvert shield.
