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?
Just to add a bit more. (using twitter gem (ruby language))
You can check if its a retweet by checking the tweet and then getting what you need from the retweeted_status hash
t = client.status(#########) #function that obtains tweet based on ID where # = tweet ID
puts t.retweeted_status? # returns true or false
t.retweeted_status # returns the actual hash for that