yep as steve said JustDecompile is definitely your friend here! the SitefinityLabel is in the Telerik.Sitefinity.Web.UI
namespace, which has a lot of the more full-featured controls such as Pager and Image.
However, I think what you might be looking for instead are the Sitefinity Field Controls
which are the controls most often used for inputting or selecting content, such as PageField, DateField, ImageField, etc. Some of these are described in this article: Selecting Sitefinity 4 Content Inside Widget Designers
and the full list of field controls are in the Telerik.Sitefinity.Web.UI.Fields
namespace. we are working to improve the documentation on this section, and thank you for your feedback.