The pending sale of a radio station usually doesn't become public knowledge until the application for transfer of the license and either a stock or asset purchase agreement is filed with the FCC, at which point it is a matter of public record. There's no FCC filing prior to that point so there would be no public record.
Prior to filing with the FCC, many such negotiations between a seller and buyer would be kept quiet so as not to upset current staff and advertisers, though that's not always the case.
I don't think there's any way to know ahead of time about a possible sale unless you have an inside source.