Unless it's written down somewhere, you can't. Comcast routers have the password written on the bottom of them, as I'm sure other service providers probably do as well. If this is a store bought router that you put the password on and forgot it, or perhaps your trying to tap into your neighbors, then unfortunately you're out of luck.
If this is your router, you can press the reset button and it will wipe the password. It will also set it back to factory settings and wipe any customization you have done.
