How do I find out what a charge is that was done on Apple Pay?

Apple Pay charges appear on your card's statement with the store or merchant identifier of where the charge was made. If they show up as "apple.com/bill", they are not charges made through Apple Pay.

They are charges from the iTunes Store, App Store or subscriptions to services and apps made through your Apple ID and have nothing to do with Apple Pay.

A charge from Apple? This is primarily a user-to-user support forum; other Apple users like you. We don't have access to information about charges to your account. Apple has a support article about charges — see each of the steps in the following instructions. They include information on contacting Apple Support if the other steps don't help. See: If you see 'apple.com/bill' on your billing statement - Apple Support