is there an app to scan gift cards to see what their worth

I doubt it. I don't know of any store that will give out technical information about how their cards are coded (none in their right minds, anyway), since that would make the card too easy to alter or counterfeit, and without that coding information, no developer could create an app that could read the balance. There are apps that help you track the balance on your cards, but it's a manual entry; the apps don't scan the card.

Regards.

usually you can call the number on the back of card and get the balance. Some also offer for you to check additional cards