Discrete Graphics not Activating
To know exactly what process is using the discrete card, look at the activity monitor energy tab "Requires High Perf GPU" column https://support.apple.com/en-gb/HT202053

To check what display devices are on your system entering the following in the terminal should work

system_profiler SPDisplaysDataType


As mentioned in a similar question https://apple.stackexchange.com/a/74881/39577