2D materials are exciting in part because they have novel properties which are often the result of the energy level spectrum. In order to investigate this, we have recently developed a new technique for assessing transport in the GHz frequency range for small flakes of 2D materials; we are presently pursuing extending this to the THz range. This is hard because focusing radiation down into such a small area, and being sensitive to conduction in a 2D material, have not been straightforward; we achieve this using a small gap in a coplanar waveguide (CPW). In addition to looking for the qualitative physics in spectral features to tell us about the energy levels, we can also accurately measure the absolute complex conductivity. I will discuss i) the basic concept and motivation, ii) the theoretical underpinning and main challenges, and iii) our results in Si:P monolayers and in MoSe2.