Customer Supplied Modem - check MAC address if already registered

I purchased a used modem from another Xfinity internet customer.  I understand that I cannot register this replacement modem to my account if it is still registered to the previous owner's account.  The seller assured me it is no longer registered to their account, but I am not 100% certain if they contacted Xfinity support to request that the MAC address be unregistered from their account.

To register the replacement modem to my account, I first need to unregister my current modem, since you can't have two modems registered to an account at the same time.  After unregistering my modem, then I must request the replacement modem be registered to my account.  It would be a terrible shame if I discovered that I cannot register the replacement modem after I already unregistered my modem.  

Can I request Xfinity to check if the MAC address of the replacement modem is already registered to another account?  Will they do that for me?  Or is my only option to attempt the registration myself after unregistering my current modem, and keep fingers crossed that it will work?