I just authorized iTunes against a laptop and I was expecting it to be my third of five computers authorized on my iTunes account. But iTunes said it was the fourth.
How do I find out what machines are authorized against my account?
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.
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 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 Allcomputers from within your iTunes Store account settings.
More information about Authorization can be found in this Apple Tech Note.
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.
It looks like there is no way to see which specific computers are authorized; you can only see how many are authorized.
Per this Apple discussion thread, you will have to deauthorize all of them and reauthorize only the ones you want to be able to play.
I'm not sure if devices count toward this limit.
To see what computers are authorized you have to go to directly to the apple website (appleid.apple.com) and sign into your account. This is different from going to your account via the iTunes app itself.
On your account on the apple website, you will see all computers including Apple TV. When I did this, there were 2 authorized computers (that I had previously used) that showed up online but did not show up when I went through my iTunes app account.