Apple care/serial number

I'm trying to register my husband's Nano because of Applecare. When I type in the serial number, and I've checked and double checked to make sure I typed the number in correctly but I get an error message that the serial number isn't valid. How can you tell if the "0" is the number "0" or the letter "zero"? That's the only thing I can figure out that I'm doing wrong - typing the number "0" when it should be a "zero" or visa versa.