How do you report someone for not claiming rental income?

To report someone for not claiming rental income, you can contact the tax authority in your country, such as the IRS in the United States. Provide as much information as possible, including the individual's name, address, and details about the unreported income. Many tax authorities have a whistleblower program or online forms for reporting suspected tax evasion. Ensure to follow any guidelines they provide for submitting your report.