Nobody “knows for sure” by looking at someone on a sidewalk. That’s a straw-man. Status is a legal fact recorded in government systems, not a vibe.
How the government actually determines it
- Records, not guesswork. Legal presence is documented: visas, I-94 arrival/departure records, green cards, naturalization certificates, parole documents, TPS, DACA, etc. DHS/USCIS, CBP, ICE, and State Department systems hold those records. An officer or ICE investigator queries them.
- Burden of proof is statutory. In removal proceedings, once the government shows the person is a non-citizen (alienage), the person must prove they were lawfully admitted or are “clearly and beyond a doubt entitled to be admitted.” People who entered without inspection or whose status expired generally cannot. See 8 CFR § 1240.8.
- Biometrics and databases. Fingerprints taken at booking, border, or visa application are matched against IDENT and other DHS holdings. Facial recognition and iris scanners are now used in the field (Mobile Fortify and similar tools) against hundreds of millions of records. Local police bookings routinely trigger Secure Communities/ICE queries.
- Most interior arrests are not random. ICE’s typical path is a local arrest → fingerprint hit → prior removal order, visa overstay, or known unlawful presence → detainer. Workplace raids, check-ins, and targeted operations use the same records plus commercial data (addresses, vehicles, utilities). They are not rounding up people because they “look illegal.”
What “illegal” actually means
A non-citizen is unlawfully present if they:
- entered without inspection and admission/parole, or
- were admitted but their authorized stay expired or was violated (overstay, unauthorized work, etc.).
That is a civil immigration violation. Illegal entry itself can also be a misdemeanor. The status is binary in law: either you have current lawful status or you do not. Temporary programs (DACA, TPS, parole) are not the same as LPR or citizenship; they can be revoked.
Population estimates vs. individual cases
Aggregate numbers (11–13+ million unauthorized) come from the residual method: Census/ACS foreign-born population minus known legal immigrants (admissions, naturalizations, deaths, emigration). That is how Pew, DHS, and others produce totals. It is not how an individual case is decided. Individual cases use the person’s file.
Why the question is usually asked
It functions as a conversation-stopper: if no one can ever be certain, then enforcement is inherently arbitrary and therefore illegitimate. In practice, ICE and CBP do not need metaphysical certainty about every person in the country. They need records, biometrics, and a legal process that already exists. When records show no lawful admission and the person cannot produce one, that is how they know.
Citizens and lawful residents who are stopped can usually produce a passport, REAL ID, or other record that ends the inquiry. People without status often cannot. That difference is not mysterious.
