There are a few different places to see if a customer has viewed their invoice if it was sent via email.

Email History

Go to the "Marketing>Email" tab.

Click on the "Sent/History" Tab.

You can check the email status under the "Status" column.

Invoice List

You can also view the status from the invoice list.

Click on the "Finance>Invoices" tab.

You can view whether the email has been opened under the "Sent" Column.

You can also click on the magnifying glass to view the invoice.

You can view the invoice history on the invoice, including if and when the email has been opened.