You can tell who is currently signed in by opening one of the applications and checking the Help menu.
Since the launch of Creative Cloud, CS6 purchases activate directly using AdobeIDs and redemption codes the same as CC, so the purchase will always be listed on the licensee's account page at but it won't have any serial number next to it. Belarc Advisor will dig that key out but it won't tell you if the serial has been registered, or if it's also being used on a second computer. Most people didn't bother, so the only records of the license key will be on the box or purchase coupon, and in encrypted format within the application's registry keys. Users who purchased CS6 in the past could optionally register their serials against an AdobeID for peace of mind but it was not mandatory.
