I don’t think it necessarily needs active enforcement. It can be as simple as:
Richy Rich: “So I claimed unemployment during my taxes, and no one stopped me! Bwa ha ha!”
Moralistic Auditor: “Wait…you did?? That’s illegal! Screw it, I always hated you, I’m going to report you to the IRS!”
IRS: “We’ve discovered you incorrectly claimed unemployment, thanks to an anonymous tip and brief investigation. Your punitive taxes have been quintupled.”
You wouldn’t always catch everyone; that’s fine, as long as the cost of abusers is not outweighed by the savings of not verifying everyone.