How to see what computers are authorized
How do you see what computers are authorized under your account
You can view the total number authorized but not what they are (other than going to each computer and checking to see if it is authorized).
The usual solution is to "Deauthorize all" and then authorize any computers that you still have and want to use. Note that only computers are authorized, not iPhones or iPads.
The following support article has instructions on how to "Deauthorize all" and reauthorize. See the "If you need to deauthorize a computer that you don't have access to" section (note there are two of those, one for Macs and one for Windows PCs) in Authorize or deauthorize your computer for iTunes Store purchases - Apple Support