You're nuts as far as construction, they are legitimate business that are paying their emoloyees off the books.
If they don't have access to drug or gambling money to launder they are paying the income taxes on their gross and still come out ahead after taxes by over 50% paying $20/hr or more.
They say the day laborers group at Home Depot and go out on small jobs. Skilled jobs such as plumbers and operators would have credentials from the hall.