It depends on the type of mistake that you made:

When filing an amended or corrected return:

Many mistakes are corrected in processing and a letter of explanation is mailed at the time the refund is issued.

If the mistake was not corrected in processing, you need to:

You can check the status of your Form 1040X (PDF), Amended U.S. Individual Income Tax Return, using the “Where's My Amended Return?” (WMAR) online tool, or the new toll-free telephone line 866-464-2050 three weeks after you file your amended return.  Both tools are available in English and Spanish and track the status of amended returns for the current year and up to three prior years.

You must enter your Taxpayer Identification Number, usually your social security number, date of birth, and ZIP code in either application to prove your identity. Once authenticated, you can view the status of your amended return across three processing stages--Received, Adjusted and Completed.  

The Web tool includes an illustrated graphic that visually communicates where your amended return resides within the processing stages. As a reminder, amended returns take up to 12 weeks to process and up to three weeks to show in the application. There’s no need to call the IRS unless the application specifically tells you to do so.