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

Balances last 5 years; inactivity fees after 12 months; some states cash out small leftovers (e.g., CA <$10) Buy low, ...

You can use a prepaid Visa gift card to shop on Amazon, but you'll have to work around Amazon's system to make it happen. Here's how to do it.