Getting a green card through marriage is one of the quickest ways for a non-immigrant to gain legal status in the U.S. Over the years the Immigration and Naturalization Service (INS) has been very vigilant on the matter, simply because the bona fides of the relationship is not objectively measured. If INS believes a marriage is fraudulent, the petition would be denied. 
To form a qualifying basis for immigration, a marriage must meet three tests: 1) It must be legally valid. 2) The couple must have had married out of a desire to join their lives together and not with the sole motivation of obtaining green card benefits. 3) The marriage must not be against public policy. 
In most cases, the marriage certificate provided by the petitioner is sufficient proof that the relationship exists. As long as the marriage was valid where performed, the U.S. will recognize it. 
If any of the couples had any previous marriages, the petitioner has to show evidence that the marriage was legally terminated. Methods of termination are death, divorce, annulment and disappearance. INS would sometimes challenge the validity of the terminations in certain instances. Remember, the whole issue of marriage with INS is largely subjective. 
INS normally assesses the divorce validity by checking the procedures and the laws of the state or country where it was done. They will also check if the couple followed the legal formalities of the state or country. 
     For proof of termination the petitioner will submit the proper certificate where applicable like death certificate or divorce decree. In a situation where termination was due to disappearance, the laws of the state where the disappearance occurs or where the abandoned spouse lives should be strictly observed. The petitioner must show proof that the disappearance caused a legal termination of the relationship. 

For instance, there was a case where the beneficiary divorced her husband of nine years, but INS found that they continued to live together as a couple. After the divorce, the beneficiary's lawful permanent resident mother filed for her as an unmarried daughter. INS considers such activity fraudulent.

    Sham marriages which keeps INS on their toes   is defined by the agency as a marriage where the parties marry primarily for the purpose of obtaining lawful permanent resident status (commonly called green card marriage).  

    Certain characteristics of a marriage may raise INS' level of alertness of a petition: huge age difference, difference in ethnicity or race and marriages where one partner commutes. It becomes difficult to convince INS that the marriage is not fraudulent when the petitioner and the beneficiary do not live together. 

    In determining if a marriage is a sham, the key focus of INS' inquiry is whether at the time the marriage took place, the couples intended to establish a life together.  The burden of proof therefore lies on the petitioner who has to show evidence that his or her marriage was entered in good faith.

    INS will conduct their own investigations into a marriage if they believe it is fraudulent. Friends and neighbors can be interviewed. INS will also interview the husband and wife separately, asking them the same questions then comparing the answers.

    Concerns for marriage fraud prompted Congress to enact the Immigration Marriage Fraud Act in 1986, a law that was ameliorated in 1990. One of the measures introduced then was the conditional resident.  With this provision anyone who petitions for his or her spouse  and gains permanent residency  within two years  of  the marriage will receive a temporary green card valid for two years. After two years, the beneficiary is granted a permanent green card, which is valid for ten years. The marriage, though, must still be in tact.  An annulment or divorce during the two years is a sure way to trigger a denial for lawful permanent status. However,  with sufficient documents to prove bonafide marriage, it can be granted.

    The conditional resident has all the rights afforded to a lawful permanent resident.