I have found this question. However I think this has changed on API version 1.1.
If I use the search/tweets method, how can I see if the tweet is a RT?
search/tweets
If it's a retweet, the tweet will contain a property named retweeted_status. To be complete, retweeted_status will not appear if the tweet is not a retweet. More info at: Tweets.
retweeted_status