Generally speaking, Metro style apps cannot get hardware details or power management state information.
See
http://msdn.microsoft.com/en-us/library/windows/apps/hh464945.aspx
You can use your own dynamic frame-rate computations to determine if the game is running 'too slow' and scale back or have user-configurable settings that let's them adjust it directly. You don't really have a way to infer 'why' and it really doesn't matter.
Users of tablets are going to want to run on battery most of time time anyhow.
You may want to review this GDC 2012 talk "Developing Metro Style Games on the Full Range of Windows 8 Devices"
http://www.microsoft.com/en-us/download/details.aspx?id=29200