- In the URL of the payment form, there will be a record ID. It is a 15 digit ID after "checkoutM?key=XXXXXXX
- Go to your instance and past this ID in the URL after "/r/"
https://apX.lightning.force.com/lightning/r/XXXXXXXXX
This will take you to the URL token record that is being used.
This process can be used to find IDs for Payment Forms and Merchant Facilities too.