system
A chat on subreddit /r/wallstreetbets
human
Gestapo. Duh. 
  
 In all seriousness though, it could be enforced much in the same way that primary residence rules are enforced. If you're filing US taxes, it's gotta be your primary residence. If you're not filing US taxes, you've got to prove residency in the US for X% of the time or show that you're actively trying to rent it out. It won't catch everybody, but it could catch enough to be a deterrent.

gpt
All you need to show is that you're 'trying' to rent it out? Sounds like a pretty easy loophole.