A lot of the albums I bought years ago were based on the recommendation of friends or in most cases I'd hear the album at their place.
Occasionally I took a chance - when I heard Jane's Addiction's Been Caught Stealin' on Rage I bought the album. As it turns out it's a f**king classic album and that's actually the worst song on there.
And there's always the bands that you know you're going to buy everything they release - Van Halen, Opeth, Devin Townsend, Alice in Chains etc.
When it comes to metal, the fraternity is generally quite knowledgeable and you can figure out what albums are good based on the general consensus quite quickly provided you know what metal genres you like.
But of course being able to download first these days makes the decision a lot easier.