A dielectric waveguide (DWG) may be used to identify a composition of a material that is in contact with the DWG. A radio frequency (RF) signal is transmitted into a dielectric waveguide located in contact with the material. The RF signal is received after it passes through the DWG. An insertion loss of the DWG is determined. The presence of the material may be inferred when the insertion loss exceeds a threshold value. The composition of the material may be inferred based on a correlation with the insertion loss. Alternatively, a volume of the material may be inferred based on a correlation with the insertion loss.