How Could I Know His Legal-Alien Status Had Changed?

An employer's obligations do not end with the initial hiring. Employers must also have practical but diligent means to track or detect when an employee's legal status changes, or if it becomes apparent that the individual was not forthcoming on their true status at the time of hire. Federal law prohibits a person or other entity, after hiring an alien for employment, to continue to employ the alien in the United States knowing the alien is (or has become) an unauthorized alien with respect to his or her employment.
Source: Inside Supply Management, http://www.ism.ws