Check if a previously owned cable modem is still associated with an Xfinity account

Concern moved here to the Customer Service help section for greater exposure to Comcast corporate employees (The Digital Care Team) for assistance.

@user_zhqovq 

I have the serial number and mac address of a previously owned cable modem. How can I check if the modem is still associated with an Xfinity account?

If you bought it off eBay, chances are it's an Xfinity owned modem.  If you got it from a friend, do you know if it was their own personally owned modem?