You can tell at a glance what sort of activity has occurred on any given asset in the asset's tile within your Project Library. You'll see the number of total comments as well as how many of your reviewers/approvers have finished what you've asked them to do (shown as a fraction of the total number asked).
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If you'd like more detail, this information is also available in the side panel of the review window of any asset. Simply open the 'Reviewers' tab to see who has viewed, commented on, and approved the asset.