Why does my MBP Retina Mid 2012 show Intel HD 4000 when it came with an Nvidia GT 650M?
As long as you have set "Automatic Graphics Switching" in 
System Preferences &gt; Energy Saver
discreet graphics get automatically switched on when you need it.
(via)

If you are interested there are applications that let you easily force the switch such as gfxCardStatus