User blog comment:Theory of everything/Grimm Speculations Part Two: Evolution and Transformation/@comment-41.160.213.42-20120608191212/@comment-1966944-20120613055327

When a Wesen becomes angry, stressed, nervous, frightened, etc. they loose control and revert to their natural form. Since normal humans do not have the ability to detect that change they do not subtly react, however a Grimm does have that ability so they react and the Wesen are able to detect those subtle clues be it a sudden dialation of the Grimm's pupils or slight facial reaction but a Wesen will know the difference as compared to their experience with other humans. I don't think a Wesen has to change when another one changes but thats more of the writers and directors decision in order to let the viewers know who is who.