How do I find out what computers I have authorized?

Go to iTunes on a computer and select your Account (on the right hand side of the window towards the top once you are signed in). Select Manage Devices under iTunes in the Cloud. All computers in this list count against the 5 total computer authorizations each Apple ID can have.

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The question asks about computer Authorization. But some of the answers here confuse Authorization with Association. I'd like to clarify the terms first, using Apple's own definitions, then answer the question.

Authorization

Authorization refers to computers only. Authorizing a computer allows you to play media purchased from the iTunes Store with a specific Apple ID. In order to use apps, audiobooks, books, DRM-protected music, and movies on a computer it must be Authorized. Each Apple ID can authorize a maximum of 5 computers.

You can deauthorize each computer individually. If you've forgotten which computers you have authorized in the past, there is not a way to list all authorized computers. Apple only tells you the number that are authorized. However, once a year you can Deauthorize All computers from within your iTunes Store account settings.

More information about Authorization can be found in this Apple Tech Note.

Association

Association allows you to download past iTunes purchases onto a different device. Apple calls this iTunes in the Cloud. In addition, association allows the use of Apple features such as iTunes Match and Automatic Downloads.

You can associate up to 10 devices with each Apple ID. By device Apple means an iOS gadget or an authorized computer. (Since every associated computer must also be authorized that limits you to 5 associated computers for these features.) So you can associate one computer and 9 other devices, or 5 computers and 5 other devices, but not 9 computers and 1 other device. You're limited to 5 computers.

There is a list of associated devices which can be found in the Account section if iTunes Store.

For more information on Association see this Apple Tech Note

So the only way to determine which computers are authorized is by trial and error: Deauthorize each computer you own in turn using Store > Deauthorize This Computer… within iTunes.

If after doing this the iTunes Store still says you have authorized computers, you only have one option: Use the Deauthorize All feature of iTunes Store, and then reauthorize the computers you own. But keep in mind, Apple only allows you to Deauthorize All once every twelve months.

Special note about about Association and Android phones.

Using Apple's own definition from the Support site.

You can't view a list of your associated devices or remove any of your associated devices using an Android phone, but Android phones do count against your ten-device limit.

More information about Authorization and Association can be found on the View and remove your associated devices in iTunes page.