Most of the differences will tend to be most noticeable with the soundstage. I'd make sure to have the speakers well away from the walls in order to maximize soundstage during the demo.
I listen for a few different things. First I listen for tone because if it doesn't do tonal information right, the rest is pointless.
For my brutal vocal test, I use female soprano. Anna Netrebko is good and has some high quality recordings. If you want to leave the room when she leans into the notes, then that's a bad sign.
Male vocals I use a korean R&B song by BTS called Singularity. On a good system this track will sound sexy and playful. On a great system you can hear past that playfulness and hear the pain in his voice.
For bass I use Billie Eilish Xanny. It's got great bass but also this 'blurry' effect that's quite cool when you listen for it. This is also a great soundstage track because there's a ton of reverb and atmosphere mixed in.
For scale, I used Mahler Symphony 5 conducted by Simon Rattle. The recording is excellent and you should be able to keep track of all the instruments, even when all hell is breaking loose.
And last, for pure ability to emote, I will throw in some Nina Simone. Usually the track is Strange Fruit but really anything by her is incredible.
