In order to report marriage fraud, you need to familiarize yourself with the laws in your country and identify the proper government organization that deals with such affairs. You will typically want to gather all of the information you can and then contact the responsible agency. This will usually be Immigration and Customs Enforcement (ICE) if you are in the US, though the Federal Bureau of Investigations (FBI) does handle some cases. In the United Kingdom, you will typically report to the UK Border Agency, and in Canada you can contact Citizenship and Immigration Canada (CIC) or the Canada Border Services Agency (CBSA). Accusations of marriage fraud are very serious in any country that has laws against this activity, so you should make sure your suspicion is valid before filing a report.
Most cases of marriage fraud are related to immigration and are often referred to as marriages of convenience. The purpose is typically to get one spouse into the country more quickly than would otherwise be possible. In the United States, these green card marriages may account for one quarter of all legal immigration over the course of a year. Many international marriages are completely legitimate, though they can also be exploited to obtain citizenship. Sometimes the resident spouse is complicit in the illegal activity, though he or she may be a victim as well.
If you suspect that people you know are committing marriage fraud, you should typically attempt to gather as much information as you can. This is not an accusation to be made lightly, and some countries will even prosecute you for knowingly making false reports. When you file your report you should have some basis for your claim, such as the spouses maintaining separate residences or other suspicious activity. You may also need to provide their names, addresses, and other important information.
You can also report marriage fraud if you are the victim. Marriage fraud is a serious crime, so if you discover that you have been a party to it then you should typically report it to the proper agency right away. Whether you find out before or after your spouse has attained citizenship, you can still report fraudulent activity. Most countries recognize that relationships can legitimately fail, though they will typically want to investigate to determine if fraud has occurred. You may also want to retain the services of a lawyer to ensure that your own rights are protected throughout any investigation that is triggered by your report.