How do you know what receipts/attachments are missing on transactions?

Let's work together, Iris. To determine how to identify missing transactions.

To check for any missing attachments in your transaction, follow the steps below:

1. Click Sales on the left panel.
2. Select the All Sales tab.
3. Click the Gear icon beside the ACTION column.
4. Tick the Attachments box.

If you want to add an attachment to a transaction, edit the transaction, then click on the attachment button, choose the file to upload, and then select Open. You can also export the list so you can have a better view of the sales transactions that don't have attachments. 

Also, I'm adding this article to help you get the most out of your financial reports: Customize reports in QuickBooks Online.

If you have other queries about transactions, please let us know. We're always here for you. Have a nice day.