Check here first
Depending on the product, you can find your serial number:
- On the surface of your product.
- In iTunes, if your product syncs with iTunes.
- On a Mac, by choosing About This Mac from the Apple menu.
- On an iPhone, iPad, iPod touch, iPod, or Apple Watch, in Settings > General > About.
If you don't have your product or it won't turn on
- If you have your product's original packaging, you can check the barcode to find the serial number.
- If you have an iPhone, iPad, iPod touch, or iPod, you can get the serial number from the Devices tab in iTunes preferences.
- You can also find your product's serial number on the original product receipt or invoice.
Source: Apple