Usually you will get an email stating you won. Or you can go to the site and look under auctions that have ended. I believe that the site is down right now as they are reversing the updates that they did last weekend. The upgrade has made the site difficult to use. If you go to thier site and read the announcements at the top of the page, they will tell you what is going on and how to pay.
http://www.shopgoodwill.com/