How can I use global search to find all posts that have been retweeted?

In the top right hand side of your monitor you can use the search bar to find all posts that have been retweeted OR you can find all the posts that haven't been retweeted. 

By typing "is_reshare:true" with no spaces it will give you only the posts that have been retweeted from Twitter. 

Vice versa you can type in "is_reshare:false" with no spaces and it will give you only posts with original content, excluding retweets from Twitter and any posts with duplicate content.