> In EM's case, I suspect he spends more time business-traveling than at home anyway, so it would be pretty difficult to prove which of his houses he "lives" in.

I'm not sure about that. I think in these high profile cases tax authorities are more inclined to look into it given how much money there is in the balance. And it's somewhat easier to track people like EM precisely because of their lifestyles: flights would be paid by his companies which would keep receipts for a few years, they would interact with other people from the company in certain location and have a shared calendar, including meeting locations.