To check which GPU is currently in command (that means which is an active VGA controller) type in
lspci -vnnn | perl -lne 'print if /^\d+\:.+(\[\S+\:\S+\])/' | grep VGA
Any controller with
[VGA controller] at the end is your currently active GPU. The others are switched off. In the following example the Intel card is active while the nVidia one is not in use:
00:02.0 VGA compatible controller [0300]: Intel Corporation Core Processor
Integrated Graphics Controller [8086:0046] (rev 02) (prog-if 00 [VGA
controller])
01:00.0 VGA compatible controller [0300]: NVIDIA Corporation GF108 [GeForce
GT 540M] [10de:0df4] (rev ff) (prog-if ff)
