In order to further investigate field emission and vacuum arcs which limit the performance of the high gradient accelerating structures. New ways of measuring the properties of the plasma formed due to these processes are being developed. A novel experiment based on a microwave probe is discussed. The HOM damping waveguides of a TD24 CLIC structure are used to insert a low power RF signal, using a transmission band centered around the 17.7 GHz dipole mode. Analyzing this transmission while the high power RF is present in the structure we can see how the plasma formed due to field emission, Multipactor and RF breakdown affects to transmission amplitude and phase, this perturbation can be correlated with the density of electrons, by means of the plasma theory.