trying to check a balance on a card purchase 6months ago

The balance on an iTunes or App Store gift card itself is either what it says on the face of the card, or nothing because it has already been redeemed. The only way to check is to try redeeming the card which of course will transfer the entire value, if any, to your account.

Redeem your App Store & iTunes Gift Card - https://support.apple.com/HT201209

View your Apple ID [not the balance on a gift card itself] balance - https://support.apple.com/HT202359

You can go to the following link to check the balance on your Apple Store (but not iTunes Store) gift card. - https://secure1.store.apple.com/shop/giftcard/balance

As I already said, if it was already redeemed it will be zero, if it hasn't been redeemed then redeeming it will transfer all balance to your account. In either case it will be zero.