It's easy. Workplaces keep employees' SSN's on hand for tax purposes. So, they raid the workplace, find and check the employee records. They call the SSN's in; employees that have fake SSN's on file, get arrested.
That would require work from the COPS. If you get arrested in a group of people I doubt that will happen. Fun with Dick and Jane (Jim Carrey version) had an example of this happening.