You’ll know that another person filed a tax return in your name if you try to file and the IRS rejects your return. The IRS will explain in the rejection that a return associated with your Social Security Number has already been filed.
Why would someone do this? In extremely rare cases, it could be accidental. But most of the time this is intentionally done by crooks who use stolen identities to collect fraudulent refunds.
Once you’re aware that someone has filed in your name, fill out and send a Form 14039 immediately. This form will alert the IRS of the fraudulent return and begin the process of restoring your identity.