We don’t have an out of box feature to show the internal values of the output cache. I developed an HttpHandler which will show the values of the internal output cache. There are many values inside the internal output cache right now we I am only dealing with the usercontrol output cache and outputcache values(mvc style) and ignoring the rest. Please feel free to extend this project.
Handler along with the demo project is under following GitHub. The demo project would display the values of UserControl Cache (Webform) and OutputCache which are internal to the .NET Framework