Chauffeur6 gave a great response in his post above. I have been involved with 2-way for many years and I have always enjoyed the challenge of tracking down interference. But, when you have a critical system experiencing serious disruption, it is not a game. It is indeed a nightmare involving long hours and high anxiety until the situation is resolved. But, having said that, there is a real thrill if you are able to locate the source and find a resolution for the problem.
I am now retired so I enjoy helping out in these situations when I can and it is more of the fun aspect when it is not a system I am directly responsible for that is in trouble.
The first step is to gather as many clues as possible about the interference and when and/or where it is heard. Next, keep an open mind about if it is local or distant. But mainly, continue to gather facts and don't jump to conclusions.
The thing that steered me in the right directon on this was being able to listen to the recording of the interference. It immediately indicated it was an on-channel and not adjacent channel signal and some fading, combined with the fact there were some ducting conditions at the time, indicated a probable distant source.
While listening for things like place or street names is usually a good clue, that was not the case here. What nailed it for me was listening to what was said and any regional charachteristics of the speakers. I did not immediately recognize it was refuse transfer stations but it was some sort of public works operation with heavy equipment or machinery. There are some subtle speech charachteristics, particularly of older natives of that area, that made me suspect that part of NY or adjacent PA. One I recognized this, the slurred KR803 started to sound more like the actual KAR803 which confirmed my suspicion of Allegany County.
Yes, I do this for "fun" or at least a sense of accomplishment here on this forum but also becuase I have great empathy for the guys experiencing the problem.