How can I check which invoices I have sent out to a customer for payment by email?

Hello there, @penny-williams-m.

I can help walk you through on how to view the invoices that were sent out to a customer in QuickBooks Online (QBO). 

The easy way to view the invoices that were sent via email is to use the Audit Log, where you can see your company's activity.

Here's how:

1. Go to Gear Icon.
2. Select Audit Log.  
3. Click Filter.
4. Mark the Transactions option as checked. 
5. Select Apply.  

For additional reference, you can check this article to learn about the audit log. 

This will get you on the right track. Feel free to leave a comment below if you have any other questions about the audit log. I'm always here to help you out. Have a good one.