The technique you need called Self Excited ElectronPlasma Resonance Spectroscopy (SEERS) and you need a Langmuir Probe connected to the system, where the dark sheath begins. If you want to measure the current oscillations (electron flow) in the plasma, unfortunately, you cannot hook up an home-brew or Dave's humble microcurent circuit connected to a multimeter and an electrode, because it will interfere with your plasma and your reading will never be a correct value.... There are some manufacturers which design and produce such spectroscopy device, which you can find, if you google.
What sort of plasma are we talking about here? and What sorth of chamber geometry do you use?