The FAQ on this topic recommends using the Usage API. But this will only show the streams that have delivered data in the last 24 hours, and will not show the definition of the stream.
Any streams you have created through the UI on datasift.com will be permanently available through the web UI.
Unfortunately, you are responsible for keeping track of any streams created through the API. If you lose track of the stream definition and stream hash, there is no way to retrieve the hash or definition, unless of course the stream was last used within the last 24 hours. In this case, you can use the /usage REST API endpoint to grab the stream hash.