In QuickBooks Online, the statuses are changed automatically once the customer received and opened the said invoices. You can see it under theInvoicespage/tab.
ClickSaleson the left menu.
Go to theInvoicestab.
It should show asViewedin theSTATUScolumn.
When your customer opens the invoice, it will show as "Viewed" under the Activities section of your invoice. You will see this activity by opening the invoice, scroll down and look for the Activitiessection. Comment back, cheers.
Let me share some clarification about emailing invoice in QuickBooks Online.
Currently, an option to identify who viewed the invoice is unavailable. You can only check the status of the transaction if one of the receivers open the emailed information from QuickBooks (see screenshot below).
Please know that your input about the feature is much appreciated. We continue to make improvements in QuickBooks Online, and this preference might be available in the future.
In the meantime, I suggest you visit our QuickBooks Blog to stay updated with all the changes to the product.
Also, I've attached some articles you can use to automatically send reminders to customers, as well as some additional information about emailing invoices: