If you are using a Comcast supplied gateway device, and if it hasn't been changed from its default value, it is on a label on the bottom. It's called the "Network Key" or the "Password" on that label.
Thanks for providing that guidance EG!
Hi bremes. Welcome to the Xfinity Forums and thank you for creating your first post. If you need further help with your Comcast Wireless Gateway rental modem, and obtaining the password/network key, I can help by accessing your account. Please send me a private message and include your full name so I can access your equipment. To send me a private, message click my name (ComcastJoeTru) to view my profile. Then click "Send a message" and private message me.
You can also find your password / network key following the instructions found on our website here: https://www.xfinity.com/support/articles/view-change-wifi-password