I gauge sound quality using three song sections. 1st=the bass guitar, drum snap and cymbals, and guitar highs and distortion at the end of Fleetwood Mac's "The Chain [Alternate Mix] from "25 Years - The Chain" compilation. 2nd=very clear bass rumble at the end of "Enter Sandman" from Metallica. Also, the boy clearly saying "My soul to Keep". "Keep" is often unintelligible. 3rd=the clarity and separation of the bar singing at the end of "Friends in Low Places" by Garth Brooks.
What to listen for?
This is aside to the obvious ones such as does a piano sound like a piano, the singer's voice sound close to them live, etc.
So, what I am trying to put together a list of songs where there is something specific to listen for. For instance, in the song Guinevere (CS&N) I have read that Crosby should sound as if he's standing in your room, front and center. On the acoustic Hot Tuna Album, they are playing in a bar and a beer bottle breaks landing on the floor - it should be sharp and sound like it's in the room with you. On Babylon Sisters there are some cymbal crashes on the left that should be crisp and not smeared. On a Beatles song (I forget which), a chair squeaks and a door opens and closes in the studio.
A good system will revel these little things. Any other that you have heard of?