Tourmaline is a gemstone noted for the large number of colors in which it occurs. Since chrome tourmaline is much more rare and valuable than ordinary green tourmaline, chemical analysis has become an increasingly important part of accessing tourmaline value. In the field, the chrome deposits normally produce similar material so if one stone tests as chrome, we can assume that all the tourmaline from that deposit should also be chrome bearing.
The easiest way to determine the presence of chromium or vanadium is by use of the Chelsea or Chrome filter. The dark colored filter will appear red upon inspection of a transparent illuminated chrome or vanadium bearing material.
A desktop spectrascope may not readily reveal any chromium content because the chrome tourmaline spectrum is a combination of chrome and vanadium and we do not see the distinct chromium lines like we see in ruby, emerald, and alexandrite.
For a more detailed analysis of gemstone chemistry (chrome or vanadium content), well equipped labs use, EDXRF; (Energy dispersive X-ray fluorescence).