Twitter has its own short url for tweets, which will show up in Google Analytics reports as referrer starting with http://t.co
To trace back to the tweet we can use Twitter's search with the url as the search term.
I recently tweeted a link to a blog post, and sure enough the subsequent day included a referrer from a URL below http://t.co
The search result can be seen from:
Chances are that this will stop working by the time you get around to reading this, as the search time range is rather limited (a few days?)