When I see used CDs or Records, I usually look at the year it was recorded, the name and songs, sometimes song lengths, the instruments used in the liner notes, maybe even thank you notes, record label, etc. Any number of factors can help me determine if it's something I would like, including the cover. I had never heard of the stoner band Sleep, but I saw their album in a store one day and the cover looked psychedelic as did the CD, and the back cover had a bunch of long haired dudes and what looked like a vague image of a plant (possible mary jane) transparented over top of it. I bought it for $2 and was happy about it. It's a bit harder with records because if they are from the late 70s and early 80s it could be any number of things from punk to new wave to shitty pop or even disco or disco rock. I am more likely to take a gamble on a $1 record than a $5 or more CD. Salvation Army prices all CDs at $2 but Value Village does it individually I suppose according to condition and if the price person has heard of them, anywhere from $1.99 to $6.99 or whatever. I have heard of a lot of bands and know a lot about all kinds of music, so if I see a name I might recognize it and know what kind of music they are and may have even heard the stuff before, if not I can tell take an educated guess on what it is. Cool psychedelic stuff from the late 60s is pretty easy to spot, the names are often condusive to the genre as are the song titles and covers.
I rarely see good used metal anything in thrift stores though, and if it's on vinyl it's going to be 80s pop or hair stuff like Quiet Riot. Fat chance finding Darkthrone or Death or Morbid Angel. People with that kind of stuff bought it for keeps or know its value (and are probably still young enough to still like it and not need to get rid of it).