There is no way to see if someone has read a message that you have sent to them.
If the recipient has turned on email notifications, they will automatically receive an email every time a new message is delivered to their inbox. Also, if there are new unread messages in the inbox, the recipient will see an alert whenever they login to their FamilyVault account to notify them that they have a new message.
It is up to the recipient of the message whether they wish to read or respond to messages in their inbox at their own discretion.