There are a few different places you can look to see if a customer has viewed their invoice if the invoice was sent via email.
Email History
Go to the Email tab. 
Click on the “Sent/History” Tab.
You can check the status of the email under the “Status” column. 
Invoice List
You can also view the status from the invoice list.
Click on the “Finance Tab“>Invoice
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.
On the invoice you can view the invoice history, including if and when the email has been opened. 