I try to stay away from the NuVinci bikes, given that I don't want to end up with a bike without a high gear.

The last few times that I did use it, I noticed if you turn the shifter to its highest gear, and there is a vague dented feel at the end, then it is likely engaging the overdrive gear. If it doesn't seem to have that feel, then it likely is adjusted incorrectly and you will be stuck pedaling away and going nowhere.