Your Amazon gift card balance can be checked via the Gift Card section of your account on the website or the app.

You can check your Amazon credit balance quickly and easily using your computer, and reload it by heading to the "Gift cards" ...

In this guide we’ll show you how to quickly redeem an Amazon gift card using your iPhone. If you recently received an Amazon gift card, you can securely redeem it on your iPhone using the Amazon app.

I am constantly receiving Amazon gift cards as a thank you from various people, which I love because I know I will be able to redeem them quickly. But sometimes the cards don't have an amount printed ...