How to detect missed screen updates

Using openframeworks with Windows I want to write an application to play frame sequential 3D videos via a Optoma projector.

As a feasibility test I am drawing alternately a large number 1 and a large number 2 to a window which I display on the projector on alternate frames.
If I then look at the screen with 3D glasses I see the 1 with the left eye and the 2 with the right eye. obviously I am using ofSetVerticalSync(true);

So far so good but if I load my PC heavily I start seeing glitches where the numbers switch eyes, presumably because my draws are not keeping up with the hardware frame rate.

I would like to know when this occurs so I can drop the next draw and thus re-synchronise without drawing to the wrong view.

I had thought that maybe I could check the current frame refresh count before I draw and if had incremented by more than 1 I could just skip the next draw. But I have searched windgi, openGL, and openframeworks and can’t find any way of getting this.

Can this be done, or is there a better way of detecting missed syncs?