All those illegal immigrants who use fraudulent documents (typically green card, Social Security numbers, driver's licenses, etc.) wonder if they will get caught and charged. Well, the answer is that the United States is so weak on enforcement that nothing every happens and there are instances of people living for decades without a hitch. Many have even taken out mortgages or have credit card debts using false papers.
So how do some aliens can get caught? Indeed, when DHS or ICE decide to do their job, this is how it typically works. Let me highlight by taking the example of Avista Resort in Myrtle Beach in South Carolina. The offices of ICE, US attorney's office, and Homeland Security Investigations (HSI) were involved in an audit of this employer (unlike in the past that dozens of armed agents will come and surround a business, arrest those without proper documents, with images of people crying and hiding their faces, now the agents might simply show up wearing business suits and will look no different than any other professionals, though, they are armed and have access to more law enforcement with the help of just one call). These agents have access to government databases on their laptops so that with a few clicks they can check if an individual is legally authorized to work in the United States. For those of you who are curious I also explained how the IRS finds out about fake Social Security numbers.
The agents order the HR Department to provide their file with I-9 forms (the law mandates an employer to have these forms on file for each employee even after they leave employment). These are forms which all employees are required to file on their first day of work. This is the form in which an employee under oath provides information on one's legal status along with documentation that proves that. The agents simply try to match that information in their databases and false papers are identified in no time. The undocumented aliens may be arrested on the spot in some cases but mostly they are tried in court (Clara Lopez, Gladys Morazan, and Cesar Esquivel were convicted and deported). If convicted they are required to serve their time and upon completion they are deported, often with lifetime ban on entering the United States. Employers of undocumented immigrants who provided no documents are also subject to prosecution but if fake documents were presented then the crime is committed solely by the alien.