How Do Macbooks Automatically Connect to Xfinity Wifi
That information is in your keychain.

Just enter the name of the WiFi in the search box in key chain.

If you do not know the name look it up.

All the networks that your computer has ever joined are saved in System Preference > Networks > advanced.

UPDATE.....Xfinity WiFi is not actually called Xfintiy, it is called Comcast.

Use Comcast when searching in your keychain.

I just looked up mine and there it is, even so I do no longer use it.