1 and 2. Governments maintain vital records. It would not be a leap for them to report changes to the IRS. 3. Mortgage companies already report mortgage intere

1 and 2. Governments maintain vital records. It would not be a leap for them to report changes to the IRS. 3. Mortgage companies already report mortgage interest to the IRS on form 1098. Not much of a leap for mortgage companies to keep track of whether a mortgage is on someone's primary residence and report that. 4. The IRS has this information from form 1098-E.