Various diagnostic methods and instruments are under development at the Forschungszentrum Karlsruhe to measure important physical quantities in the accelerating gap of high power diodes on KALIF with a high spatial and temporal resolution. The methods include optical spectroscopy, refractive index measurements, dispersion interferometry, and high resolution energy analysis. The setup of these diagnostic tools and the first results obtained for applied and self-magnetically insulated diodes are presented. (author). 6 figs., 5 refs.