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.
I am not a Comcast Employee. I am a Customer Expert volunteering my time to help other customers here in the Forums. We ask that you post publicly so people with similar questions may benefit from the conversation. Was your question answered? Mark the post as Best Answer!