If your coparent has not opened a message you've sent them, it will be marked with a Never Viewed status.
When your coparent has opened a message you've sent them, it will be marked with a Viewed date and time. You can find this information in the "To:" field when viewing a message. Additionally, on the OurFamilyWizard web application, the message's status symbol will change from a filled circle to an empty circle.