In the Illini Union Bookstore on Wright and Daniels, they can tell you your i-clicker serial number. Just take your i-clicker to the bag check-in and tell them you need to know your i-clicker number, and they will help you. It's quick and easy.
If you have registered your i-clicker in the past but can no longer read/remember your i-clicker serial number, you can visit this link and get your i-clicker serial number by suppling some of the information you used when registering:
http://www.iclicker.com/registration/lookup.aspx