If you want to get the best reference (that I'm aware of) to judge the stereo imaging of your system there is none better, IMHO, than a compilation of cuts from several recordings on the Opus 3 label. Title 'Depth of Image'. This was originally on an LP but it is still available on CD (check out Amazon). Each cut is different - small band, solo instruments, small groups, voice, etc. It comes with a description of each cut, i.e. what you should hear. On a quality system, properly set up, the sound is spookily holographic. Give most folks trying to optimize their imaging a real challenge! Kept me busy for a few years. :-) This record does not deal with proper speaker set up, there are several others out there for that, this really about fine tuning.
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?