My only idea would be to show a grid on one eye of the HMD, and look at a real object with the other eye. By comparing the size of the grid in the HMD to how large a measurable object appears, you should be able to estimate FOV, right? FOV as in http://en.wikipedia.org/wiki/Visual_angle. Given the distance from the eye to the object, and how wide/tall the object is, it should be possible to estimate, right?
Any input is welcome
![Idea :idea:](./images/smilies/icon_idea.gif)