If an employer has 100 employees and 90 of them are illegal it's pretty much obvious he knew he was hiring illegals.
Did you know that employers check against a Social Security number database that is likely to show that those 90 numbers are all valid? What is an employer to do after SS numbers check out?
Do you know an employer can be sued for refusing to hire someone because he suspects they are illegal, especially if they turn out not to be?