How do I find out my Apple Pay balance and how do I pay it off?

There cannot be any pending balance in Apple Pay. Apple Pay is not a credit mechanism, only a payment conduit.

Apple Pay uses your credit and debit cards that are added to it for payment. Payment is is immediately extracted from them when it is used.

The only place you'll see and owed balance will be on your Credit Card's monthly statement like you would if you had used the card outside of Apple Pay.

If using Apple Card, you can see your balance and pay it by following the steps here: View your Apple Card balance and scheduled payments online - Apple Support

If using a 3rd party card form your Bank or other card issuer, then the balance would be only on that cards monthly statement, and nowhere else.

I assume you mean Apple Card?

Your current balance is in your wallet. Select the Apple Card, you'll see your balance and current payment due.

Pay your balance in your card (tap the payment due amount) or at card.apple.com