Most routers will show it in their configuration pages.
Be sure to read http://forums.cnet.com/5208-6132_102-0.html?forumID=32&threadID=235558&messageID=2413798#2413798 titled "Is WEP dead?" to avoid an unpleasant surprise.
Recently a wired/wireless network was installed in our home by a professional networking company. We recieved straight lines to the desktops, but for the laptops we needed wireless. When they installed the Belkin router they added WEP/WAP encryption to it. But now I would like to add another laptop I am re-formatting and need to install drivers, but I do not know the key code. Is there any way I can find the key?