I'm surprised the article doesn't delve more into how to interpret what you're hearing. For example, recordings should sound different from one another on great speakers. Bring along material that you know sounds different. If the speakers being demonstrated make the recordings all sound bright or dull, or suddenly every selection has a very pleasing bass richness, that's a good indicator that the speaker has probably been "voiced" to demo well with some well-chosen bumps in the frequency response. On really good speakers properly set up you can hear the differences in the studio or venue acoustics.
I agree with the article that well-recorded jazz is excellent test material. Trumpets should "blat" at you. A trumpet is actually a loud, in-your-face instrument. A stand-up bass, properly recorded, is a very subtle instrument, and shouldn't sound like it's 20 feet tall. Cymbals are awesome test sounds. Many good speakers these days can get the sound of wood hitting brass correct, and most people recognize what that sounds like. On the other hand, lots of tweeters can't reproduce a cymbal properly without it sounding hashy, and those speakers should be avoided.
Some jazz singers are recorded very naturally; Diana Krall is a good example. On a very good speaker she can sound live on several of her recordings.