(That is to say, someone told you about it, you heard it on the radio, or someone made you a mixtape.)
Search engines didn't exist. Independent fact verification didn't exist.
What did exist? Networks of indie zines, of semi-pro zines, of fan centered magazines. Flyers, bulletin boards, the guy at the counter at the video store or record store (who was often a sexist prick, but like you win some you lose some.)