The latest Nvidia drivers are giving problems with G-sync. I think it depends on the game/GPU.
In mine I think that g-sync is active everytime my FPS drop from 100fpz (I've got my panel OC'd from 75hz to 100hz), but fps still go up from 100hz. Older drivers did not show this behaviour. Lets hope new ones correct this issue, since a lot of people are complanining and Nvidia must be aware of it by now.