Nvidia has the following to say on How to know if G-SYNC is enabled:
To verify whether G-SYNC is enabled for your game, click Display
from the NVIDIA Control Panel menu bar, then select Show indicator
Note: Depending on the type of monitor connected, the menu item could state Show indicator for G-SYNC, Show indicator for G-SYNC
Compatible, or ...
While the answer by @LaserJesus might be more robust and relevant for more people, I'll list mine as well cause it's an easy fix. My monitor has a power saving mode and a performance mode. Switching from performance to power and back to performance worked for me (and has worked a few times before). Adding here cause this is the first thread that comes up ...
This problem seems to have fixed itself. I was running with nomodeset for a while and the latest update seems to have fixed the issue. Please refer to my answer on NVIDIA Developer Forums for more information.