Option 1: Notify members of content updates
When content that a client has either a) created, b) is following, c) has contributed to, is updated; clients will receive an email and portal notification. If none of the above applies, when you update the content, you can choose to Notify other group members so that they receive a notification.
Option 2: Account administrators can check the audit trail
You can also use the Audit Trail feature in your account settings to see what content has been viewed, when and by whom. In here you will first see the action that has been taken e.g If a member has sent a file, to see if the client has opened/viewed the file, you need to see if there is another action with their email address. If their email address is on show, this means they have viewed the file.
Option 3: Request approval from your client
You can also request for a client to approve a file, in order for them to confirm that they have seen it and are happy with the version.