If you know how to go into the routers internal settings then you can go to the security tab and view the password if it's different from the password that you use to get into the router.
If you want to know what the password is for your homegroup network then that's in any of the computers that are on the network.
Can you be more specific as to what wireless password you want ?
1)Open the web browser that you use: Firefox,Google Chrome,IE,or Opera and type in the IP address for your router,and press the Enter key. http://www.routeripaddress.com/< Router ip addresses can be found here
2)A window will open and then type in your routers default username and password,and press the Enter key again http://www.routerpasswords.com/< Router usernames and passwords found here ***** Username and Password may also be located on the side or bottom of your router *****
3)Navigate to the Wireless Security\Network Security Settings Tab and open to locate or change your WEP or WPA Security Passphrase. Use WPA because it provides better security.
4)Write this Security Passphrase down and put it in a safe place to access when needed.