First, you must find them.
How?
Welfare,Social Services, Multi-Lingual Translators, PUBLIC SCHOOLS, Emergency Room attendance (have INS at the emergency rooms used most often), Profiling (They ARE Criminals, after all...), Employment records, Re-Certifying Soc Security numbers and addresses, those "Check Cashing" clearinghouses, cease ALL $$$ transfers to Mexico unless ID is verified, and they have a valid Green Card/Visa...I'm just warming up!
What's you answer besides a welcome mat and more welfare $$$ from MY pocket?