The only way to insure people are here legally is to enforce the law.
How can the law enforcement community enforce a law when you have cities like Portland,OR, Chicago, IL, New York City and, the state of California issuing drivers licenses to undocumented workers?
How does a policeman check, in any state?
Well, you start slowly but, surely state by state issuing federal ID cards until every state has been covered. Then and, only then you can be sure the people here are legal.