how do I tell how much credit is on an apple gift card

An Apple iTunes Gift card can only ever have the full credit it states or nothing at all.

Once redeemed onto an account the gift card is emptied and the balanced transferred to the account.

The only way to know, is to try to redeem the gift card code again.

You can view your current Itunes credit balance on your account by following the steps here:

View your store credit in the App Store or iTunes Store - Apple Support

An Apple Store (hardware) Gift Card can have partial credit: See here for how to get its balance:

Apple Store Gift Cards

Hi,

Follow the instructions in this support article > View your store credit in the App Store or iTunes Store - Apple Support

At what stage are you in the redemption process?

As the article you attached above states (near the top), once you redeem the card, the value will appear and be associated with your Apple ID.