Marriage fraud is an ongoing concern for U.S. immigration authorities, namely U.S. Citizenship and Immigration Services (USCIS) and the Department of State (DOS).
At one point in time, they believed that the majority of marriage-based green card applications were fake, just scams to get a green card. That belief got debunked, but ongoing concern is fueled by the incontestible fact that green cards based on a spousal relationship are one of the few avenues to getting a green card that do not require job skills or a preexisting family relationship.
For the most part, the type of investigation that the U.S. immigration authorities conduct in order to expose fraudulent cases is fairly superficial. Because of time and resource restraints, they primarily rely on applicants to do a convincing job of providing documents proving a shared life and passing an interview (plus follow-up fraud or "Stokes interviews," in doubtful cases). Most approvals and denials are made based on these factors alone, without follow-up.
Nevertheless, U.S. immigration officials' powers go far beyond reviewing documents and talking to applicants within the confines of their office.
When in doubt, U.S. immigration authorities can conduct background checks by searching the Web and checking on the applicant's credit record, using the Social Security numbers given within the application documents.
If they choose to, they can interview the applicants' employers, parents and other family, and friends, visit or schedule interviews at their homes, and so forth. They would usually take such steps only in cases where fraud is suspected.
Note, however, that a suspicion of fraud could arise in marriage cases that, while valid, simply have not been well-documented by the couple, or where one or both people are forgetful or bad at answering questions about their life and marriage.
What's more, U.S. immigration enforcement authorities have been known to use a tactic referred to as "bed checks" in some regions of the United States. This involves immigration officers showing up at the home where the applicants claim to live together, typically early in the morning. They see who answers the door. They also ask to take a look around, in which case they'll be looking for signs that both people sleep and keep their clothes, toothbrushes, towels, and other items of daily living in shared space within that residence.
Bed checks have actually been reported both before and after approval of the immigrant's green card (lawful permanent residence).
People can legally refuse to admit immigration officials conducting a bed check. Such refusal is, however, likely to lead to further types of investigation.
For more information and assistance with applying for lawful permanent residence based on marriage to a U.S. citizen or green card holder, see the Marriage-Based Visas and Green Cards section of Nolo's website or consult an experienced immigration attorney.