I assume you mean Apple Cash as Apple Pay is not an account that holds any money. If you mean something else, you will need to explain what that is.
For Apple Cash and sending messages to people in Messages, all you need to do is look at the status of your Transactions on the Apple Cash card. If not received by the person you send Apple Cash to within 7 days, it will automatically be returned to you. If the status shows Pending, they have not received it yet, and if it shows Completed they have received it.
The money is sent to the Apple Account used for that person and that is where it was received. Anyone that uses that persons Apple Account is able to receive the money, and another reason why Apple Accounts should not be shared with others. There is nothing to distinguish different users with the same Apple Account.
More information about sending money with Apple Cash and issues that may arise are here:
https://support.apple.com/en-us/117946#cancel