Tell them that it was a mistake for Google not to put a red light indicating that a photo or video is being taken, and that you hope there will be one in future iterations (even though evil folk may find a way to somehow disable the light).

WSJ: Google Glass: An Etiquette Guide

After what felt like an eternity waiting, the little red light on the camera flickered, indicating that the image was ready to view.

BBC: Meet July??s BBC Travel Photo Nomad