"Just out of curiousity, how do you police illegal immigration without some form of I.D.?"
That's an excellent question, and one that doesn't get raised often enough. In reality, there's not much of a way to enforce those laws without some recognizable form of I.D.
Social Security numbers don't work, since they can be obtained pretty easily, really, and the Social Security card isn't an I.D. anyhow, since it doesn't include a photo or any fingerprints or any such stuff.
A birth certificate is useless, too, since you can get one by sending a request for it to the local county clerk. A quick trip to a local newspaper archive for the year you were born will pop up a number of names you can use to request a birth certificate.
Driver's licenses....well, they'll do, as long as the process of getting one involves some sort of proof of citizenship. But you can see the two paragraphs above for ways around that.
How DO we tell who is a citizen and who is not? That's a tough question, and I don't have a good answer for it.
Recent naturalized citizens have their naturalization papers, even if they only speak English haltingly. I have my passport, which should serve.
And exactly how can you get one without showing some form of ID to the clerk? They do check that nowadays.