You probably can't. If you can get to the admin screen of the router, you can check to see if the passphrase (if you used one) and/or the keys are visible. Otherwise, from the admin screen, you can reset it.
If you can't get to the admin screen, you'll have to do a hard reset back to factory defaults (a small button somewhere on the router, usually requiring a stylus to push, that you have to hold in for a time - say, 30 sec). Doing this will reset ALL of the settings on the router, including logon information to your ISP (if applicable), SSID broadcast and name, etc.