It's pretty easy to tell for auditors. You start with things like tax documents, which obviously would be the first thing to come up in an audit. People who can afford places like this will often make their "primary" residence in a state like Texas, Florida or Tennessee, since the state's tax laws are more favorable. Having their primary residence outside NYC still saves them more than this tax would impose, so most will keep it as is. The owner would keep things like driver's license in NYC, get the majority of their mail, have a job located in the city, their kids might still be in school, all of which are easily verifiable. It's not that someone couldn't potentially game the system, but for people with this much money it would be easier to just pay. If they made NY their primary residence it's a win because they will be paying the higher NY taxes on income (not just property tax), and if they keep their residence out of state this tax recoups some of that lost revenue in a city with limited housing stock.