The simplest way, examine with XRF gun (X-ray fluorescence)

The cathode ray is just a beam of electrons.

The color of the fluorescence depends on the substance in the target.

P. A Pella has written:

'The development of potential thin standards for calibration of x-ray fluorescence spectrometry' -- subject(s): Fluorescence spectroscopy, Thin films

For wavelenghth dispersive X-ray spectroscopy adequate crystals for light elements (with a higher wavelength) are more rare and good.

B. J. Chamberlain has written:

'The analysis of oxide systems by x-ray fluorescence'