This cannot be accomplished with iTunes. All you can do is open iTunes on each one of your computers and authorize / deauthorize on them individually. The only other option is "deuthorize all" which can only be performed once per year. After you "deauthorize all", authorize each one of your computers individually.
In order to see which devices are authorized on your account:
Click View Account
Enter your password
Look in the second section iTunes in the Cloud
To the far right you'll see Manage Devices
Once it's clicked you will see all of the devices currently authorized in iTunes/on your account
I was going to say, "Hope this helps," but based on the above, I'm pretty sure it did!
For anyone who may come across this thread, Tina is incorrect. That doesn't show all the systems that are authorized to a given account, only those which are set up for automatic downloads or redownloading, which isn't the same as those that are authorized to an account to play protected content. There is no way to see a list of those systems which are authorized to the account.