Can I see if my customer has opened the portal?

This article tells you what the status of your email means.

With a sent email, you can find an indication of whether the customer portal has been opened or not. When you go to the page of an invoice, customer, or case, you can open the email you want to know about in the timeline on the right. To do so, click on the blue 'Email' link.

A pop-up will open with the email preview. Here, you will see the 'Status' of the message.