You've come to the right place. QuickBooks Online offers a section in the transaction history of every Invoice where you can ensure it sent properly, just follow these steps:
Checking Invoice Sent Status
From QuickBooks Online, navigate to the Sales tab and the Customer section. Click the name of the customer whose invoice you're checking.
Find the Invoice in the list and click to open it. The Filter (▼) menu can help you narrow the list if needed.
Click More at the bottom and select Audit History. If there are multiple sections, expand them by clicking (▼). Verify the status in the Sent and Last Sent field.
I'm including a brief video demonstration below for your convenience:
This allows you to view the historical information of whichever transactions you need. Check out our detailed guide on audit logs in QuickBooks Online for additional information. Please keep in touch with me here should you have any additional questions or concerns. Thanks for reaching out, wishing you and your business great success in all of your endeavours.