how do you determine who is legal and who is not?
It's a two-pronged effort. "Multiply redundant backup systems" type thinking.
how do you determine who is legal and who is not?
First, you implant ID chips issue ID cards (sorry, I got a bit ahead of myself) to everyone to ensure that there is no way for a "non-legal" to avoid being identified.
Then, you make everyone legal. [rimshot!]
Go ahead and laugh -- but then realize that I'm not joking.
I forgot the punchline (to my non-joke).
"What you say? We end up with everyone legal, AND everyone saddled with invasive, privacy-raping biometric National ID?"
(Um, that was the punchline.)
See how easy it is to impose this sort of totalitarian nightmare on a "free people"? A little fear, a little threat, a little promise of safety and security -- in exchange, as per the Standard Bargain -- for your FREEDOM -- and there ya go.