Pull any bill out of your wallet. See the dark green numbers on the top-left and top-right corners? Those are serial numbers, ...

If you have your original packaging, the serial number is printed on the barcode sticker on the box. If you don't have the box, you can find it by going to your Apple ID page in a web browser.

A radar serial number reads the same backwards and forwards, like a palindrome. For example, 06288260. A good condition, uncirculated $1 radar could sell for about $25 — or even more than $100 ...