You know, there are some interesting difficulties in this whole lack of a uniform system of identifications. Let's take this case:
1. The person is not a citizen.
2. He/she speaks excellent English, as do many people outside of the USA.
3. He/she never attempts to vote.
4. He/she never applies for any sort of welfare.
5. Through some subterfuge, the person has a SS# and Driver's Licence (it's not that difficult).
6. Most importantly, U.S. Citizens are not required to present identification on demand.
How do you identify this person as an illegal alien? The answer is that you don't. He/she lives and operates as a legal citizen, and probably will never be discovered.
Well, if they're not voting, not collecting welfare, paying taxes (that's why they have the SSN), and not committing other crimes, I don't have as much problem with them. Set up a system to legalize that exact status, with a valid I.D. to confirm it, control the border, and you've got a solution. No freeloading, no crime, no voting, and paying taxes.