How do you know somebody uses your SSN?

Question

specially to file to IRS.

Answer 1

Check your credit report.  Go to AnnualCreditReport.com to access all three credit bureaus online.  Get a report from one bureau every 4 months to spread out the reports thru the year.

If accounts show up that aren't yours, dispute them.

Answer 2

The first clue might be if the IRS holds up your refund and/or sends you a letter saying you did not report all of your income (more than bank interest you may have overlooked) when you know you did, or that you already filed when you hadn't.

Of course if you are inexperienced selling property, stock, etc. it could just be your mistake not properly accounting for cost basis.