how do I find my password for my xfinity router/modem?
I, too, use Comcast. So, did Comcast provide the router or did you?
If Comcast provided, then silverliquicity gave you the right answer as long as the default password wasn't changed by you. If you changed the password and don't remember it, you should contact Comcast Customer Support.
If the router is yours and you don't remember the password, you can reset the router per the link WR2 provided.
Otherwise, discussions on how to bypass security measures are not allowed on Tom's Forums as we have no way to verify you are the legitimate owner of the network in question.