How do I find out which computers are Authorized to use my iTunes account?

You need to manually check each one you think might be authorized; it's possible that a computer is using multiple authorizations. There is no list available from your end.

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There is no way to associate a specific computer to authorization for iTunes.

Authorization

iTunes Store- Authorize or deauthorize your Mac or PC.

In iTunes you use the Authorize This Computer or De-authorize This Computer option under the Store menu in iTunes' menubar. For Windows use the ALT-S keys to access it. Or turn on Windows 7 and 8 iTunes menus: iTunes- Turning on iTunes menus in Windows 8 and 7.

Deauthorization

You can de-authorize individual computers, but only by using those computers. The only other option is to "de-authorize all" from your iTunes account.

1. Open iTunes on a computer

2. From the Store menu, select "View my Account..."

3. Sign in with your Apple ID and password.

4. Under "Computer Authorizations" select "De-authorize All".

5. Authorize each computer you still have, as you may require.

You may only do this once per year.

After you "de-authorize all" your authorized computers, re-authorize each one as required.

If you have de-authorized all computers and need to do it again, but your year has not elapsed, then contact: Apple - Support - iTunes - Contact Us.

For more information on authorization and de-authorization: iTunes Store- Authorize or deauthorize your Mac or PC.