For rewarded video and in-stream video campaign, you can check video view completion data from the dimensional report.
Enable "Show Video View Completion Data"
When you get report, choose Inventory Apps/Sites, you will be able to see CompleteViews and VCR data of each app/site.
Complete views: the number of views of the visitors who watch the full video ad
VCR: Video Complete Rate, the percentage of Complete views/Impressions.