On the Home tab, select Editor. In the Editor pane that appears, scroll down to Similarity and select Check for similarity. When the check is complete, Editor shows you how much of your content matches text that it found online (indicated as a percent), and the number of distinct passages in the document for you to review. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.