[dragonwriter]

> The law is such that it is illegal to employ someone "not eligible to work", so it is really up to the employer to enforce the law.

Actually, it is illegal to employ someone without the employee and employer both completing the relevant portions of the I-9 form, which requires the employee to present to the employer certain documentation of eligibility to work.

In addition to investigations and enforcement actions stemming from information raising suspicion of a violation, the government can conduct audits (administrative inspection) of I-9 compliance, which need not be based on any suspicion.

[jsprogrammer]

According to the I-9 form, submission of required information is voluntary, though an employer may be later be applied penalties if they are unable to prove that they determined eligibility.