how I see payments already applied to a invoice

Thanks for joining the Community, JimmyG31b.

You can view payments applied to an invoice by reviewing its Transaction Journal.

Here's how:
 

1. In your top menu bar, go to Customers, then Customer Center.
2. Locate the customer and double-click their display name to access their profile.
3. Find an invoice, then double-click it.
4. From its Reports tab, select Transaction Journal.

You'll be able to see each payment that's linked with your invoice.

You'll also be able to find many detailed resources about using QuickBooks in our help article archives.
 

Please feel welcome to send a reply if there's any additional questions. Have a lovely day!

So I have to do several steps to see what was applied to a invoice instead of one click on a icon BRILLIANT! SMH

@JimmyG31b 

You can see the payments in much the same way in newer QuickBooks versions:

Click on the Reports tab at the top of the Invoice and then click Transaction History.

Which, as it happens, is not a report, so one wonders why Intuit put it there? Perhaps the designer was unclear as to what a report is?

@ZackE 

RE: From its Reports tab, select Transaction Journal.

The Transaction Journal button does not show the Transaction History window. It shows a Transaction Journal report, which does not in any way help the customer find the requested linked payments. Which you should know.

RE: 2. Locate the customer and double-click their display name to access their profile.

There is no reason to double click the customer name to access their "profiles". Which you should know.

And, that is not what a customer record is called in QuickBooks. Which you should know.

RE: 3. Find an invoice, then double-click it.

You cannot "find an invoice" when editing a customer record. Which you should know.

Do you use QuickBooks?