How much does that really happen? In the emotionally charged debate over immigration, it's hard to say.
But as one step to stop illegal hiring, the Obama administration is reportedly asking for $112 million in its new budget -- a 12 percent increase -- for E-Verify, a massive computer database that employers can use to make sure the people they hire have the legal right to take the job.
"Companies want a way to be legal," said Tamara Jacoby, president of ImmigrationWorks USA, a group that represents employers on immigration issues. "We're not giving them the tools to do it."
E-Verify, currently run by the Department of Homeland Security, is used by about 120,000 employers around the country. The system is voluntary, and there is no charge for companies to log on and check out job applicants.
If a worker tries to get a job, an employer can go online, enter the person's name, Social Security number and other data, and find out whether the applicant is in the U.S. legally. The government says 2 million checks were run in 2006, the last year for which complete numbers are available, and preliminary answers come back in 3 to 5 seconds.
At least, that's the idea. In reality, companies, government officials and advocates for immigrants' rights say E-Verify needs work.
"If you're looking at it as an immigration enforcement tool, this is not the answer," said Tyler Moran, Employment Policy Director for the National Immigration Law Center, a group that says it stands for the rights of low-income immigrants.
Moran and others say the system is not up to the task of screening thousands or millions of people, and employers often misuse it, either to get around government regulations or because they don't understand what they're doing.
"Right now it's only used by a bit more than 1 percent of the country's employers," Moran said. "That's pretty small. We don't know what would happen if it were expanded rapidly."
Rep. Steve King, R-Iowa, the ranking Republican on the House Immigration Subcommittee, defended E-Verify, arguing that if illegal immigrants know they'll be stopped when they apply for jobs in the U.S., they'll have less incentive to sneak into the country.
"In these tough economic times, no American jobs should go to illegal immigrants," King said in a statement. "E-Verify is a quick and easy system that shuts off the illegal worker jobs magnet and provides employers with a way to verify that their employees are legal to work. Americans cannot afford to lose more jobs to illegal workers. The American people want this Administration to enforce existing immigration laws and protect American workers by using the E-Verify system."
Few companies contacted by ABC News were willing to talk on the record about their experiences with E-Verify. For instance, the giant poultry company Perdue sent the following statement :
"We are doing all that the law allows to verify each applicant's identity and employment eligibility. If we find that an associate has presented false information on an employment application, that person will be immediately terminated."
Perdue confirmed it uses E-Verify, but referred questions to Jacoby at ImmigrationWorks USA.
"There are still glitches in it," Jacoby told ABC News. "That said, some system like this is absolutely imperative for a working immigration system."
As the administration expands the system, the pressure is on to make it work -- a fact that Homeland Security Secretary Janet Napolitano acknowledged in Senate testimony on Wednesday
"I believe E-Verify is very important and must be an integral part of immigration enforcement moving forward," Napolitano said. "So we know that, with incentives and otherwise, E-Verify can really make a difference. We are committed to making it better."
Ferdous Al-Faruque, Jason Ryan and Theresa Cook contributed reporting for this story.