How can I tell if my Contact has opened the email I sent from Matrix?

Matrix can only confirm whether or not the email was successfully sent from its mail server- it will not tell you if the recipient received or opened it.

However, you can use the Last Portal Visit column to see the last time they logged into their Portal.  If it is after you sent them the email, you can essentially infer that they did receive and open the email you sent.

Go to My Matrix> Contacts.

Scroll down to the desired Contact and look at the date in the Last Portal Visit column.  That date represents the last time they accessed their Portal (by clicking the link in a Matrix email).