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
Make sure your "re-tweet" is not actually a quote of an another tweet. In this case, I had to use the quoted_status field to get the original tweet, rather than the retweeted_status one.
quoted_status
retweeted_status