How can I see everything authorized to use my Apple ID?
If you login to www.icloud.com with your AppleID you can go to Settings and look under "My Devices" to see all systems (Phones, iPads, Macs) authorized and logged in to that account.

This will also allow you to sign out of any other browsers signed into your account.

I am not aware of any way to see any information on what particular computer is logged into your account via iTunes (PC or Mac) but you can "De-authorize" those computers all at once. That can be done from iTunes that is currently signed into your account.