Hopefully this tool works correctly, but if you get an error message, don't get too worried just yet. I got an error message when I put in the full number, including the number in the box. When I put in that number in the box, it said my process was undetermined.
To verify that you are eligible for the Nintendo 3DS Ambassador program, enter the serial number in the box below. The serial number for a Nintendo 3DS is located on the bottom of the system - just below the barcode - and starts with the letters CW followed by 9 digits. (Be sure to include the last number in the box.)