Waveguide arcs were a constant problem, and detecting the exact location was sometimes difficult. Automatic arc detection was great at protecting the equipment by initiating an "early off"; however, the source wasn't an exact science.
An old trick I've used on previous programs was to cut a length of nylon tubing about 18 inches long. By putting one end in my ear (after removing any sharp edges) and sliding the other end against the waveguide, I could hear the ping created by a single arc. As I got closer to the source, the ping would get louder. This was great for isolating waveguide connector, dummy load and harmonic filter problems.
I learned this from an auto mechanic who used this to trouble shoot an air leak on my car engine many years ago.