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?
As everyone else has mentioned, you can check to see if the retweeted_status property exists in the response subfield for that Tweet.
However, per the current version of the API, every Tweet object has the field retweeted that stores a boolean value (True or False) that will tell you if a Tweet has been retweeted.