I was doing pre-inspects on a couple houses yesterday afternoon.
This is often what happens when I walk into an under-construction house:
There look to be about 5-6 Mexicans working out of an old van finishing up insulation and starting drywall - I can tell that somebody has been cooking food in the garage.
When I walk into the house, one of them sees me and vanishes into the family room, 3 or 4 of them leave the family room through the garage entry and get in the van. One comes inside the foyer to watch me and the other keeps working. I just nod to the one watching, he nods back, and I do my inspect and leave. (As soon as I leave the others will go back to work.)
In my experience it goes like this: there is only one English speaker, and probably only one or two "legals" in the crew. They are one of several crews, often family members, who are hired by one guy contracting with one of the builder's subcontractors (this insulates the builder from any immigration questions or legal responsibilities), who takes a big chunk of the workers' earnings in turn for letting them work for him.
The same organization (foreman or farm boss thing) happens with the Koreans, except that most of them are legal. The Koreans tend to do more intricate, time intensive labor across several trades, and the Mexicans do most of the grunt-type construction work.
This happens all over, in all good-size cities. Those of you who live and work in smaller cities may understand the problem, but probably don't grasp the size of it...
As far as I can tell, there is never any enforcement of immigration law, never even any questions asked by anyone.
The only time you hear of "raids" or any inspections, are with factory employees. Factory situations where illegals are employed are a very small percentage of the illegals working. I venture that most illegals are employed in construction and restaurant/hospitality and itinerate farm-work - although in the south, I have read that a lot of them are employed in the trucking industry.