Hiring an alien for employment if you know that the alien is unauthorized is unlawful. The law defines an "unauthorized alien" as one who is not lawfully admitted for permanent residence or who is not authorized to be employed by either the INA or the Attorney General of the United States.
One way to avoid problems with the law is to hire a U.S. citizen. A U.S. citizen is not an alien, much less an unauthorized alien. However, be careful about how you determine whether a person is a citizen. Asking the wrong way could still get you in to trouble. (See below.)
2: Hiring an "authorized alien" keeps you compliant
Another way of avoiding problems with respect to unauthorized aliens is to hire authorized aliens. One type of authorized alien is the permanent resident - the holder of the so-called "green card" (although they have not been green for many years).
Another group of authorized aliens are those who are in a status that allows them to work under certain conditions. For example, foreign students, in F-1 status, can work a limited number of hours. In these cases, such aliens should have form I-766, an employment authorization document (EAD).
A third group of authorized aliens may work without an EAD because of their particular non-immigrant status. Examples include those in H-1B status. Such aliens instead should have, and should present to you, a form I-94 or form I-94A, with an endorsement and reference to their status. The I-9 form and instructions will provide guidan