There is a little "envelope" icon next to the profile picture. If it looks like an unopened letter then it hasn't been read yet.

If you are not already in the email section then go to My Profile scroll down to My Email and click. From there you will be at your Private Message page and your Inbox will be open. On it's menu bar is Outbox this is the sent mail folder.

Once you click on Outbox you will go to the mail you sent. You will notice that next to the profile picture is a little envelope icon. If the email has been read it's triangular "flap" will be up, or open, and this means the person you sent it to has read it. If it is not opened and looks rectangular then the mail has not been read.