At the time I was travelling extensively while railfanning so I was able to verify many of the items personally. I also contacted people in many areas to see what they could verify. Some stuff was gleaned from FCC records, but heavily edited and massaged.
For the Illinois book a couple of us travelled the state extensively and verified almost every agency, especially at the county level. That was a huge amount of work but a lot of fun!
RadioReference was not a thing then, at least not to the level it is today.
