How do you check a used cable modem to make sure it is not connected to someone else's account???

Thank you very much for reaching out to us here @milton_7. The only method we would have to check on the status of a modem would be to check on the MAC ID of the device. In order to check on that I would need you to send me a direct message with the full name and complete address for your account. 

To send a direct message [private message]:

   Click "Sign In" if necessary

   Click the "Direct Message" icon or  https://comca.st/3IKBEoU

   Click the "New message" (pencil and paper) icon

   The "To:" line prompts you to "Type the name of a person". Instead, type "Xfinity Support" there.

   - As you are typing a drop-down list appears. Select "Xfinity Support" from that list.

   - An "Xfinity Support" graphic replaces the "To:" line.

   Type your message in the text area near the bottom of the window

   Press Enter to send it