The web app not only tells you the people who have stolen your tweets (to the right of your tweets,), but also the people you have copied the tweets from (to the left of your tweets).
To use it, simply enter your Twitter username and hit Go. The app will analyse your last 50 tweets. To analyse more, you will need to pay. A tweet will be defined ‘stolen’ if it has a similar content to one of your tweets without mentioning your twittername.
However, the app doesn’t analyses all the last 50 tweets. All tweets with mentions and all tweets with less than 16 symbols are excluded.