How do you determine if the "illegals" are actually illegals.
The same way the identity of bail jumpers is confirmed.
Apples and oranges. A bail jumper is a known identifiable person who has committed a crime. You can look for that specific person and arrest him based on reasonable suspicion that the person looks like the bail jumper, acts like the bail jumper and smells like the bail jumper. As far as a potential illegal alien, it is not reasonably known that the person is an illegal alien until you identify him by asking them for "his papers".
How else would you do it??
It would work.
You have not demonstrated such.
An illegal leaves an obvious trail of clues: drives, but no license, works, but the employer can't verify citizenship, has no bank account, prepaid mobile phone with no fixed address, etc.
Any PI could reliably sniff out 90% of illegals. And heck, because I'm a nice guy, any illegal that has succeeded in buying a house can stay. They got hustle.
The fact remains, we DON'T need to be a surveillance state.
What is the result of building a surveillance state? It's military recruitment that is 8000 below quota. Our intrusive government can go to hell, or the bureaucrats snooping on me can fight for their own gravy train. I don't care, and my kids ain't goin'.