Hi JSej!
Welcome to QuickBooks Community. Let me show you how to run a report that shows invoices with billable expenses.
When you run the Transaction List by Customer from the Reports menu, you'll see the billable expenses with transaction type as Billable Expense Charge. Click the small Gear icon next to the Export button and check the Invoice Date box. If it shows a date, it means that the billable expense has been invoiced already. From there, you can easily determine which of the invoices this billable expense has been linked.
Also, you can create and use custom fields moving forward. You can enter a tracking or reference number in this field to easily find out if this invoice has a billable expense. This custom field can also be included in reports such as Transaction List by Customer by adding it from the small Gear icon.
Lastly, here's an article if you need more help in running, customizing, and saving a report: Run reports in QuickBooks Online.
Reply to this post anytime if you have follow-up questions with reports. I'll assist you!