I've got your back, @SCH LLC.

You can open pull up the Transaction List by Customer report to see which invoice the credit was applied. Here's how:

1. Go to the Reports menu.
2. Scroll down to the Sales and Customers section and click Transaction List by Customer.
3. Once opened, set the desired date and then click the Customize button.
4. Tick the Filter field and check the Transaction Type box.
5. From the drop-down choose Credit Memo.
6. Hit Run report.
7. Look for the customer you want to check and click the Credit Memo.
8. You'll see the 1 payment made button, click that and select the date.
9. You'll be routed to the Received Payment section and from there, you'll see which invoice the credit was applied. 

Also, to get a list of all payments and invoices for each of your customers, you may pull up the Invoices and Received Payments Report. Then, customize it so it will only show the info you need. 

Let me know how this goes and post a reply below. I'm always here to help you see your payment history. Have a good one.