A lot of radio stations surveyed sales at local record stores. Not a perfect method, but if you had stores you could trust, you could see what was getting big without your airplay, what albums were selling (which smart PDs realized was often people responding to single play by buying the LP) and even oldies that were suddenly selling again (exposure on TV, movies, at some event)----and what records weren't moving even after weeks of airplay.What was the common research method before the 70s?
And, of course, before call-out research, there was call-in. The request line. Not the best statistically, since only your most active listeners were likely to call in, but stations with large enough audiences (and in some cases, people specifically assigned to answer the requestline so the jock could concentrate on his or her show) could use those interactions to ask a question or two about what else that listener liked, what they were burning out on, etc. Trouble is, asking people who already listen to you what they think is not a great way to grow a station beyond perhaps longer listening. The advantage to call-out research was the ability to talk to your competitors' listeners, find out what they saw as the strengths and weaknesses of their first choice, and find ways to exploit that information.