If you are using a Comcast supplied gateway device, and if they haven't been changed from their default values, they are on a label on the bottom, back, or the side.
There are many ways to view your Network SSID if you are using Comcast equipment. If you do have Comcast rental equipment, I would recommend visiting View and Change Your Wireless Settings.
It providse varies methods of viewing your network such as using the X1 Remote, XFINITY My Account Application. the stick on the side of the modem, or logging into the user interface.
If you are using an owned modem and router, different steps would be needed.