How do I find any expenses that have not been marked as "Billable".

Hi there, @steve304.

Let me help you identify which bills are billable or not in QuickBooks Online (QBO). 

You can run the Transaction List with Splits report to view these data in the program. Simply customize its columns and filters to see the non-billable bills. Here's how:
 

1. In the Find report by name bar, search the Transaction List with splits.
2. Click on Customize.
3. Select the Row/Columns and put a checkmark beside Billable.
4. Click on Filter, then choose Bill as a Transaction Type.
5. Select Run report.

Now, you'll be able to see which expenses aren't marked as Billable. To bill them to your customers, simply click the non-billable transaction to open its window. Then, check the BILLABLE box and click Save and close.

For more detailed information on how to customize reports, you can check out this article:  Customize reports in QuickBooks Online.

I'm also attaching these articles for future reference:

• Enter, edit, or delete expenses in QBO
• How to remove a billable expense charge

If you still have other questions in mind about viewing your bills in QuickBooks, don't hesitate to comment below, @steve304. I'm more than willing to help. Have an amazing rest of the day!