I found a post by a Dell employee at Dell's support forum that explains it very well:
Dell Inspiron n5520 comes with multiple video cards. You will not be
able to know which GPU is active using the ‘Catalyst Control Center’.
This feature has not been provided by AMD.
In Muxless hardware design, the switchable graphics will use
appropriate video card to assigned application from the catalyst
control center and process the graphics of the application and give
the output through the Intel’s integrated video card so using any
application on windows you will not be able to see the discrete video
Therefore, when you are using graphic intense programs, (Example:
Playing games, video editing, photo editing) the discrete graphic card
However, during normal usage, if you are viewing applications like
office, power point or word documents the integrated graphic card
The same post then goes on to explain that you have some control over it via the usual ATI/AMD Catalyst Control Center software. It also offers some remedies for situations where options may not appear in the CCC.