Agreed, with a couple of caveats.
In a number of industries, a businessman who refused to hire illegals will be quickly out of business, as his costs will be significantly higher than the competition. Either he operates at a loss, or he has to raise his prices, which causes him to lose customers to the competition. If the law isn't enforced, a single employer cannot take a stand by himself.
I am not aware of any effective way for an employer to tell if an applicant is illegal. Asking questions in this area is a great way to fall foul of the discrimination laws.
Are you kidding??? All you have to do is ask them to speak English. If they can't, you know you have an illegal on your hands.